4850 issues please help

ok I'm at wits end. I purchased a 4850 as requested by popular demand to upgrade my pc. I finally got it installed and updated the drivers, and it will not run cod waw at all. I was at least able to play the game at before with my **** card now I can't play. It either crashes constantly or if i do manage to get in the 2x it happened it's sluggish and I get black square artifacts. I have dumbed down my system to where it looks like windows 2000 nw not xp pro. hasn't helped. I unistalled the game and reinstalled after updating drivers and directx again for the 10th time. It runs cod 4 flawless and handles all other applications with ease. It just won't run cod waw.