Having problem with geforce 6200 (in World of Warcraft)

Ok so I built this new computer
specs:

Asus K8N4-E Deluxe
AMD Sempron64 2600+
XFX GeForce 6200 TurboCache 256mb
512 Kingston RAM
WD 200gb caviar 7200 SATA

HD is OK, ran lots of diagnostic stuff. RAM's ok too, everything is fine.
I'm running winXP pro with all updated drivers, DirectX 9.0c and etc, everything runs ok but WoW...

What happens is that a minute after I log, wow starts to alt+tab to windows, and then alt+tabs back to wow, then back to windows and back to wow and so on.... This is driving me mad!!
Everything gets stuck...

I can manage to close wow and stop the alt+tabbing but thats in a right time when it gets back to the desktop and I have like 3 seconds to right click wow and close it.

After that, if I dont restart my computer, I can't play any other games. If I try to get back to wow again, it will freeze and restart my computer... with no BSODs, just freezes and bam, restart.

The attachment is the only error in the event viewer I can find so I *think* its the thing when I try to run wow a second time after it alt+tabbed to hell.

Any insights? I know this GPU is crap, it has only 64mb and takes the other 192 from my RAM, anyways, it should work. My old system has a geforce2 64mb mx400 and it can run WoW with no problem

I'll try running 3DMark to see if it locks my computer or something, and will try to induce that restart again to see if it generates any other event error and post it here.

Thanks in advance!

Tried to load WoW after the first crash and It won't let me, see attachment for the error message

OK now I installed 3dMark2001SE.
Made a basic Benchmark, with default settings, with no problem, no errors at all.
Tried again benchmarking with max settings (AAx4, triple buffering, 1280x1024, it ran the test and ended it allright.
Then tried the Demo option, minimal settings, and it ran ok till that matrix-like part, it froze, then crashed, with this error: (attachment)