Do you have the latest drivers for your graphics card installed?
-
NVIDIA Drivers
-
ATI Drivers
Also make sure you have the latest
Windows Updates installed.
If you have more Doom 3 related questions. Why don't you try ask it at a forum about that subject.
-
http://forums.newdoom.com/
-
http://www.forumplanet.com/planetdoom/
etc.