STOP 0xc000021a Fatal System Error: Session

When you decide to download/install XP Service Pack 2, you should be aware that it conflicts with the changes they may or may not make ntoskrnl.exe

If such replacements to ntoskrnl.exe exist, then it will render your computer into a reboot loop with the above error not displaying (by default: You can change XP not to restart on system failure by way of disabling - "Automatically Restart on System Failure" - in System Advanced options.)

It's highly suggested, for the upgrade or install, to disable or uninstall such programs.

Cheers.

I didn't know TweakUI changed XP's executables. I thought the only thing it did was to change registry settings.