Powercolor HD4850 problem, please help

So, I got myself a new computer, and it keeps freezing when I play a video(oddly enough, never when I use videolan, but freezes when I use zoomplayer, ms classic media player), moving things from the hdd and when i'm downloading things.

The pc has NEVER crashed in far cry 2 or crysis(even not after 4-5 hours playing), only when I'm downloading something and/or watching a video in zoomplayer/ms classic player
I've tryed 2 different hdd's, run memtest for 5 hours without problems, 3 differnet windows XP (sp2, sp3 and 64) and it happens on all of them. But when I tryed anohter gpu card in the pc, it stopped freezing when downloading/watching movie.

This is what happens when it freezes:



is the graphic card defect?

any help would be much appriciated

Other speccs:
Corsair TWIN2X 6400 DDR2, 4096MB CL5,
Intel Core 2 Duo E8400 3.0GHz, Socket
Corsair Powersupply 550W Black, ATX/EPS,
MSI P45 ZILENT, P45, Socket-775, DDR2,
PowerColor Radeon HD 4850 "PCS" 512MB,
Samsung SpinPoint F1 640GB SATA2