There seems to be some issue with ATI.
Some german guy reported this:
http://r4d1um.seitzinger.eu/DSC00038.JPG
Ati Radeon X1250 with latest drivers
AMD Turion 64 X2 TL-60 2x2,0 GHz & 2 GB Ram
Windows XP SP2 (32 Bit)
If he starts the game it looks like that. If he alt-tabs out and right back in it fixes itself. But as soon as the next map is loaded it is garbled again and he has to alt-tab out and in once more.
He encounters the same in GL Quake, GL QuakeWorld and Hexen II.