From the menu, go to Controls (under Skirmish), then select the top option, Mouse Configuration. You can change the mouse sensitivity for both vertical and horizontal movement here.

thanks for your reply :-)

unfortunately those sliders aren't fixing the actual problems:
aiming left is more sensitivity than aiming right, and aiming up is more sensitive than aiming down.

aiming down has an actual dead-zone:
if i mouse my mouse downwards slowly, then my aim will not move at all. there's a minimum speed you must move your mouse downwards at, in order to have your aim move at all.

and about left vs right; let's say if i move my mouse 5cm to the left, and then 5cm to the right, then my cursor will not be back at where i started. in stead, it will have been moved to the left. meaning that the sensitivity for aiming left is actually higher than the sensitivity going right.

those sliders in the mouse configuration do not fix the difference between up and down, or the difference between left and right..

:-(

Sorry if this is a silly question, but are you sure it is not just a problem with your mouse?
I read this and tried it out for myself but I cannot seem to find this 'dead zone' and do not notice any difference in sensitivity when moving the mouse in different directions.

Do you possibly have a controller plugged in? That could be interfering.

i don't. i looked into disabling joystick/controller support in AvP, but there doesn't seem to be a way to do that. after some googling, some people have suggested that these mouse issues have something to do with added/improved controller support in one of the later patches. but unfortunately that information was of no help to me, heh. i don't think that's a silly question :-)
but i sincerely doubt that it's my mouse. it's a good mouse (Logitech G400), and this is the only game in which i've experienced these specific problems. i have no software or drivers installed for my mouse, and no drivers for my controller either. so i also doubt it's a software-related problem.
i've found that (at least) a few more people have experienced these problems(having a dead-zone and inconsistent sensitivities), but no-one seems to have found/posted a fix :-/

@Grzegorz925
can i ask what mouse and settings you were using?
(acceleration in windows disabled or enabled? mouse software installed? dpi on mouse? sensitivity in windows? sensitivity in avp?)
because i can imagine that it could be hard to spot these issues if you have acceleration in your mouse, or otherwise turned on in windows/software. the same goes for using a high sensitivity.

me, i'm using 800dpi, 6/11 windows sensitivity, enhanced pointer precision = off, and my sliders in AvP are at about 1/4th of the bars.
if you can be bothered, i would really appreciate it if you could make sure you have acceleration disabled and use a (very) low sensitivity, and check again if you experience this dead-zone if you move your mouse downwards [i]really[i/] slowly. just push your mouse downwards with one finger, at the slowest speed you can do it at ;-p

as for the difference in left and right; i put some heavy books on the left, right and bottom of my mouse. then i made sure my mouse didn't go up (by mildly pushing it into the bottom book), and then i just moved the mouse to the left until it hit the book, and then to the right until i hit the book.
i set it up like this to make sure that my mouse only went left-right, and not up or down at all. and this way i was also sure that my mouse was moving the same distance towards the left as when it moved to the right.
and the longer i repeated this, the more to the left my aim was..
so whatever the problem is, it's certainly not my imagination ;-p

anyway, thanks for the replies guys :-)
and sorry for the wall of text, i just would love to get to play this game again

I use a trackball, Logitech m570, and have never had any problem like you describe. I've tried to recreate what you describe but could not. I would look for a hardware/driver problem. Maybe installing a proper driver for your device would help.

thanks for the suggestion :-)
though, i don't know where i should start looking. because in many dozens of other games, i've had no issues with mouse movement whatsoever. so how could a lack of drivers be the problem, if no other game has problems with me not having drivers? (well, technically i do have drivers; the default windows 7 drivers :-) and i can't imagine them causing problems.)

and thanks for checking it in-game.
i'm starting to wonder if my game could somehow be confused by certain settings or software on my PC, and is in turn using the wrong code to read my mouse input..
but i just can't think of what could be causing such confusion.

i fear i might be doomed to either skip this game or play with broken mouse controls :-(
well, thanks anyways guys.

I cannot give you a full list of settings as I do not know how to find a lot of this out :P
But I am just using a Microsoft Basic Optical Mouse v2.0 (nothing flashy) and generally play the game on maximum (in-game) sensitivity. I tried out your in-game settings, putting the sliders at around 1/4 of the maximum, it does feel that looking down is slower than looking up. Looking left or right however feels very much the same.
I also tried looking for this 'dead zone' at this sensitivity, and you are correct, it does seem to stop randomly when I am slowly moving down. However I am not sure if this is entirely what you mean.

I've been looking at this in my codebase for Bucake and have managed to find a fix for the deadzone issues (dead zones on moving down and to the right)

I thiiiink these were present in the original 1999/200 release? It's certainly something in the present in the original source code but might not have been an issue due to the compiler they used back then.

thanks for stopping by :-)
i've done some testing in AvP Gold, and the dead-zones are present there as well. thank you for having another look, i appreciate it :-)
these 'stops' are indeed a result of the problem. apparently the code for (mouse) input is a little bit of a mess.

if you are not experiencing these issues, then i guess you can consider yourself fortunate. because there's no escape from the code for any of us! :-)

@Grzegorz925
if you care to 'know for sure', turn both sensitivity sliders all the way to the left (just hold leftarrow until you can't go any further), and then tap rightarrow [i]once[/]i. be sure to press 'use these settings' to save them. this will make sure you have the lowest in-game sensitivity.
i bet these settings will make you notice the dead-zones. you might not even have to move your mouse all that slowly :-)

i've also noticed that the sensitivity of horizontal mouse-movement goes down when you walk/crouch. in my opinion, that's a bit annoying. i've sent sirlemonhead an e-mail about this, and he's replied that he'll take a look.

edit: i'm not sure if vertical movement even uses the same algorithm as horizontal movement. i'll try to do some testing on it..

I am also having problems with mouse movement. I don't know what you call it but it seems the movement on certain areas of the screen "stick", so maybe it is deadzone?

WOW!!! I am shocked so many people don't notice this problem.

Everyone has this problem as it is a problem with the game. The dead zones have been in the game since it released. If you go back and play the origonal version of the game you will find the same problem. The dead zones are the same in the Mac version.

Sirlemonhead did some tweaks to his AvP d3d9 patch but the dead zones remain, though the mouse aim did improve slightly.

Worse still the dead zones change when you crouch or zoom in and the mouse sensitivity is different for each species.

Here is a bit I wrote to Sirlemonhead.

"The mouse aim dead zone is only there for moving the mouse forward, and for moving the mouse right. There is no dead zone for moving the mouse left or down. This problem seems to be the same problem Shadow Warrior and Duke Nukem 3D have. In those two games mouse aim is slower when moving the mouse forward, and also slower when moving the mouse right than it is when the mouse is moved left or back. Eduke32 has fixed this problem for Duke Nukem 3D I believe.

I also found some other quirks. V-sync slows mouse aim down significantly. Turning off V-sync causes the mouse aim to become extremely sensitive. I am betting that would also be the case in the steam version since the steam version does not allow V-sync to be turned off. The Marine's ammo counter on the pulse rifle does not work when V-sync is off. And I was very surprised to find that mouse aim sensitivity differs between species. The sensitivity is higher when playing the predator or the alien, even though the setting in the controls menu has not changed. This is true for both the steam/gog version of the game and your renderer. "

There is something severely wrong with how the game uses mouse input.

I have been looking through some of the source code by Sirlemonhead. But I don't really know what I am doing or where to start. I've just been looking at some bits of code in various files. I saw what tweaks Sirlemonhead made, there is no problem there I can see. I am not sure where to look there must be something buried/scattered deep in the source code.

Or I wonder if since the build engine games exhibit similar behavior, could it be something wrong with what is used to build the game code, some bugged library or something?

I wonder if this fan made patch works with the GOG version? Seems like they fixed that mouse issue.

[url=http://homepage.eircom.net/~duncandsl/avp]http://homepage.eircom.net/~duncandsl/avp[/url]/

That is the D3D 9 patch by Sirlemonhead. It does not fix the dead zone issue but does improve things as I mentionend and is the code I have been peeking at.