Hmm.. I'd be curious how the motherboard would accept other video cards; similar 6200's of the same or even different brands. Have you tried much with the new GPU once it gets to Windows, confirming it works correctly and doesn't give you other errors? And power, I find it rather doubtful, but have you tried another power supply? I know the card's a fairly low-power one, but with the computer's age it might be worth checking out if possible...
Aside from that, I can't really think of much else either...