setting lighting to medium has fixed it for some ppl
the problem occures when there is not enough ram available
- you can try to increase the page file size
- deactivating visual themes in xp is probably not as effective as in vista but still might help a bit
- go all the way to low settings for testing purposes
note: there are already alot of threads regarding this issue - maybe you can find better help in there
also: this is prettry pr related since it rarely happens on vbf2 (uses alot less ram) therefore this forum would be right for you too (search in there):
PR Support - Project Reality Forums