system
6
Your booting (in Safe Mode now) problem could be related to your steps:
I tried repairing XP using my original installation disk.
You had SP3 already, so, although less frequently seen, an old XP CD (pre SP3 for you) might mess your booting process. “Little” things have change during the years, and the booting areas (MBR and PBR) and booting files might have changed.
You need to try a repair of XP booting process with an updated CD. The same goes to the system files checkup.
If you manage to boot into Windows Safe Mode, uninstall Avast. You might want to use the Clear Uninstall Utility from Safe Mode http://www.avast.com/uninstall-utility.
If you manage to boot into Safe Mode, but for some reason you can’t uninstall Avast, at least try disabling it. In Windows Services, look for Avast Antivirus and change the start method (from the current “auto”) so it won’t start. Then using msconfig, don’t allow avastUI.exe to start.
Only then you might try booting in Normal Mode, since Avast won’t start (nor it service, neither it UI) so to avoid the BSOD.
Then you can download the newest beta version (which should solve your BSOD).