Open Question: Computer Help!! username and password required after service pack installed.?

So I don't know the exact details because this is my gf's computer. She said that windows installed SP3 and then needed to reboot. When it did Windows asked for a username and password to login. She never needed a password before to login. Could some boot files be missing or corrupt? I've never heard SP3 causing something like this. She has Windows XP Pro btw. There is no repair disc so I can't use that to repair Windows if that is the problem.