Sorry for my english i'm french ;(
try this, i had this problem on WIn XP with Geforce 2 TI leadtek with MSI mainboard with VIA chipset.
I have install 21.83 detonator, via 4in1 and i have in BIOS VIA.
Put off the AGP *4, i have put *2 and i works but power decrease but i work (only 50img/s on Unreal tournament)...
But on win2K with 21.83 with AGP *4 it work (110 img/s)
it's AGP VIA/XP nividia incompatibility