I was trying to do an in-place-upgrade to fix some issues. This did work before, but now I'm encountering this error.
There's just too many reasons for this to occur that it would be pointless to try to troubleshoot. You are better off either doing a fresh install or using the updown8 trick with an install disc.. At least with the updown8 trick it will put any potentially important data in the windows.old folder for you to fish out if you need.