I'm going to guess that it's something I hit last week - SP3 upgrade issue with OEM XP builds.

Try this:

1. On the screen that comes up, hit F8.
2. Tell it NOT to restart on fail.
3. Get the stop code on the BSOD (blue screen of death).
4. Google this: "stop <code you got> wxp site:microsoft.com

You should get a page that explains the issue and suggests a fix.
For me, it was boot into safe mode (thankfully that worked) and make a registry change.

The issue as reported affects OEM builds with non Intel processors. Have an AMD chip in there?