Haven't used my computer for 3D games etc for a week or so now, but since me and my mate brought Dungeon Siege 2 I took my system round to his to play, but as soon as I ran it I noticed artifacts on the screen. Just flickering blocks of pixels a similar colour to the surrounding pixels. I ran the dxdiag tests and at first it had problems but then I disabled my antivirus and it ran fine (without any artifacts), I ran the artifact tester and got a score of 124 on average setting!
Just to point out I haven't even attempted to overclock this card. The temperature stays around 37 degrees C.
Here are my specs:
Windows XP PRO SP2 (Direct 9.0c (along with DXSDK))
1.3Ghz AMD Duron
LeadTek 5950 Ultra 256MB (TV capture edition)
The only thing I can think of is low power, but I've taken out everything I don't need