I have Windows XP and I got the same problem. All I did was that I went and grabbed the "OpenGLsetup" off of the net and installed it. Has been working beautifully ever since.

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http://kevbocat.homeip.net/alliedassault.jpg

I figured out the solution for the problem running the game (or demo) in XP with a
nivida chipset.
First:
Update to the latest Detonator drivers for your NVidia card for windows XP/2000.
( http://www.nvidia.com/view.asp?PAGE=windows2000)
Version 23.11

Second:
After the install of the videodriver search
for these 2 files:
opengl32.dll (670Kb, dated 7 sept 2001) and
opengl32.lib (325Kb, dated 16 july 1996).
The can be found in the windowssystem32
directory.

Third:
Copy these 2 files into the game dir
x:Program FilesEA GAMESMOHAA
(x is your drive letter where the game is installed!)
overwriting the old files that were there.
(the old opengl32.dll is 717Kb)

Last step:
Remove the '3dfxgl.dll' from your game dir.
that should fix the problem. I believe the
engine tries to load the 3dfx drivers
first. I think when they did the RTCW code
they did not do a validity check to make
sure there was not another opengl driver
before erroring out.

After these steps MOHAA will work on XP too!!

Have fun people!!