rlpw, I had problems with my old 200W power supply when I switched from a 650Mhz Duron to a 1200 Mhz Athlon yeeears ago, and you're running an Athlon XP with a more power hungry graphics card than those that were available at that point in time?
Computers are weird.
Hm, could you borrow a power supply from somewhere to try?