I had the exact same problem, and here's the diagnosis and how I fixed it (or intend to fix it, for part of it).
XP tries to detect major system changes, which usually require "activating" windows again. When you put an old HD on a new mobo, it F's with Windows and windows thinks it needs to be activated for a new computer.
I'm using a barton 2500+ and I attempted the same overclock. My bottleneck that caused the problem was my dual-channeled 256mb pc2100 chips from Kingston (old memory, i was broke). They took the 200 fsb kick pretty well at first, and ran dual channel 266. When windows tried to boot, the computer soft-rebooted and it detected as dual channel @ 254. After that, I started getting messages like what you got, or "IRQL_LESS_THAN_OR_NOT_EQUAL" errors. I booted in debug so I could see where the system was hanging and it was at the Amd AGP driver, which munches ram like a biazznatchgazziza. Since I already knew I had a memory leak (as if Kingston doesn't leak like a seive anyway) I figure that the memory trying to detect new hardware for Windows, load the AGP drivers, and do so trying to catch up with the bombardment of information from the fast proc with open FSB confused it all to hell and crashed it. When I pulled the OC off the problem went away with the memory leak but windows was now corrupted, and I had to reinstall it. MAKE SURE YOU FDISK THE DRIVE BEFORE YOU REINSTALL or the windows install might still be corrupt when you're done.
