I was trying to install Heroes Chronicles Conquest of the Underworld from cd on my mothers Windows XP computer-it always installed fine in the past. Now when I try to install it, it gives me both an error about direct x and that there is not enough space on the drive (but it has 92 gb free) and that it seems to think it's Windows 2000 instead of Windows XP. As mentioned, this has never happened before and I've installed the cd's on her computer just fine. The only difference recently was that I tried to run one of them on my Windows 7 computer, so I tried a different cd and I got the same error. Any ideas?

Well, do you have it installed? Known issue with Heroes 3 and 4 installers, as they can't differentiate MB from GB. Ignore it.

Sorry, but if NOTHING had changed since you installed in the past, what you installed then would still be there so why are you installing again? If you (or your mother) had de-installed your Chronicles game to make room for something else, then maybe that something else is what is giving you problems.

I've never understood how the different levels of DirectX relate to each other, whether you can have more than one at a time on the same system (different ones for different games), or how you discover which the active one is.

Windows XP turning into Windows 2000 - that is weird.

Unless you cloned one CD from the other, we know that it's not your CD at fault. My suggestion is to check your disk for errors and try again if there were any. If not, run your defrag utility (I'd give the name, but I haven't used Windows in almost 20 years.) and see if that helps.

Not sure how it should look for original CDs. I have russian re-editions CDs and last time i use theme was 15+ years ago., i use GOG package 8in1

(it is still available by library download API link if you find it or if you manually change download ID - look to GOGdb for additional info and direct links to GOG APIs where download links present)(ofc it is same links as in Library and you need game on account to download, there is no direct links to files anywhere and not having unbandled games return same "no access" error for bundle as for not having games).

Probably Win2k is normal. It also can say WinNT, also normal.

Try to pre-made destination folder. Exact this path
D:/Games/HeroesChronicles-1/
No secial symbols, only latin letters and digits, no spaces and even __ - use minus instead.
Path should be short. But mostly I can use for old games old installers long folder names too. So actually game root folder probably can be Heroes-Chronicles-MM-1.

Using more folders and real full names - that one actually bring issue. It is not even installer but WinNT and NTFS limitation. Path luke this is too long too many characters
X/Games/Might-n-Magic/Heroes-3-Chronicles-1-.../
Although skip last part with subname of H.Ch. episode name probably no issue. Issue can be or not in additional sub folder for game universe series. At other hand, Might-n-Magic-Heroes-Chronicles-... is too long for one folder name, better split to two folders.

So. Try to create temporar folder. Install it to one folder and then move to another. Although that probably require some Win Registry tweaking after (which one letter if CD/virtual drive and where originally installes is not matter for DRM). Or maybe not. Most needed argumns trere recreated by game at next launch, if remove previous. CD version DRM still can look to CDpath and InstallPath.
Of course if you want to use one of the path i mention here with full name and refer to MnM universe. Moving to such folder can be done after installation.
Or ignore this section, look to very first path i wrote above.

Large TB storages are not issue actually. It is path issue only. And issue with installers ability to create folders. They more limited in lenght/characters than premade folders allow to use. Using installer avoid use even minus. D/Games/HeroesCh1/ is maximum possible variant within installer.
Or more likely D:\games\heroesch1\
WinNT (and some versions of Win, 98/Me only maybe?) dont care about backslashes and capital letters, while old installers do. If you want capital letter for HeroesCh1 folder - rename it after installation or pre-made before setup.