problems installing windows 2000/XP

I have a system with ABIT VA6 motherboard, Pentium III 866MHz, 256MB PC133 SDRAM and GeForce 2 MX (32MB) graphic card.
I had a Celeron 566MHz before, and since there were mainboard BIOS updates that added the support for Pentium III processors up to 1.1GHz, I decided to buy this 866MHz processor and to improve my system. After I have installed the new processor Windows 2000 couldn't boot (there was a message to try to repair installation - it didn't work or anything else I've tried). So, I tried to clean install Windows 2000, but I have never succeeded again. I am not able to install Windows XP too - there is the same problem. During the GUI part of installation a blue screen shows (page fault in non paged area error or registry error or some other error message) but it repeats after every restart. I have the latest BIOS installed. I've tried to install with a different graphic card (ATI Xpert 2000), but there is that same message. I've also tried disabling AGP2x, Video shadowing...