redacted; didn't see the second page of this forum with the answer when I posted this.
still, odd for gog to sell a game without giving us a way to get past copy protection without searching the forum, for such a major game.

The keys are in the directory of each game.

Mmm, it is so hard to look in the game folder for manual, clues, or wahtever? Also, the game creates a shortcut to the keys.pdf it's in documents/keys.... Hard to guess also...

Maybe it's not hard to guess, but guessing shouldn't be necessary. Seriously - if it's actually impossible to remove the copy-protection (despite the idea of GOG) then it at least needs to be made very clear in about as many places as possible (webpage, download page, installer, manual) that you have to look in the keys file. Expecting people to "guess" it on their own isn't very professional.
I'm not saying this to criticize GOG, which does an amazing job at bringing us classics to play again. Perhaps in this case it just could not be removed or perhaps (since it's only after playing the game a little while that it pops up) they just missed it. It happens. But people here like Lobuno being sarcastic and insulting to people for not figuring this out - not cool.

Well, I apologize if that sounded too harsh... no insult intended, but IMHO there's no need to guess anything at all, it should be very obvious to everyone, if you have trouble with the game, at least consult de docs first, but saying something like "odd for gog to sell a game without giving us a way to get past copy protection without searching the forum" it's very far from truth... actually it's completely false.

Just some warning guys:
Print the keys page before playing. I was playing for about an hour, then the game asked me the password, and I alt-tabbed to see the Keys.pdf file, and it crashed. Yep, I think I just wasted one hour of my life now.

You must have gotten unlucky. Whenever it's asked for the password, I've been able to alt-tab or use the windows key with no problem. Still, printing out the codes is handy. It's annoying that the damn thing asks more than once. I can understand Clouds and Darkside asking separately, as they are two games combined (did it do this in the full talkie CD version too?), but each surely shouldn't ask more than once.

Don´t use alt+tab, use alt+enter to get to window mode. This never crashed for me (alt+tab has many times).

You can enter the wrong codewords as often as you want without negative consequences. So you can enter wrong codes and save afterwards to make sure you don't have to replay the game if it crashes while looking at the codes (never happened to me).

Alt-tabbing mm4&5 in vista gives no issues, in winxp however it crashes. I've edited the dosbox config file to play it windowed, then all your problems are solved. Makes it easier to keep a guide-website handy too, because all the different terms in de game can get confusing (static objects, flaming whatnots, clever thingies, etcetera).

I.m alt+tabbing in MM4&5 as I write this with no issues at all in Windows XP. (Also alt+enteri ng)

I wonder if they tried locating the matching strings and replacing them with NULLs? The game is likely written in C and therefore would use NULL terminated strings and in most games that checked words NULLifying the string would just let it match anything. (Of course if they were clever they could use a debugger to see where the check routine was and just have it always branch to the correct entry return regardless of what word was enter... granted the check would still appear to be there, but should take any entry, so it'd just be a minor slowdown/annoyance...)

Just want to point out that modders already KNOW how to remove the CP from MM4+5... There's an editor floating around, and you can use it to remove the CP events from things.

That's odd that the copy protection is still there in 4/5. I"m currently playing the GOG version of MM3 and it automatically skips the protection. The window briefly flashes before continuing to the start/load screen. I would think it would be as easy to bypass in 4/5, considering they are just about the same game as 3.

Here's a way to remove the copy protection:
http://www.patchery.net/patchery/patch.html?id=1000013