Creating Password

I have two accounts that were pulled over from my existing xp install. They
did not have passwords but now i want to add them. Every time I try to add
the password the system creates a critical error and reboots 60 seconds
later. I have created a new account and added a password and it works fine
but the original ones do not. Any Ideas? I am a firm believer in not
installing any new OS over an existing but thought I would give it a whirl.
Maybe it needs to be said that no upgrades just clean installs.