I had to help my wife figure out why she suddenly could not login to her computer. Her regular account just said that the password was incorrect and the Guest account said something like, "the user profile service failed the log on." This made me think something must be wrong with Windows' entire user profile service.
I Googled around and eventually tracked down the Windows 7 install disc to try and do some command prompt magic to kick this thing into shape. As I was typing in some stupid command in which I had little confidence, I noticed I missed the letter 'd' in the word administrator. So I backed up to add it back in, pounded on the 'd' key into the correct place and lo and behold nothing appeared.
Protip: Make sure each key pressed actually shows up in the password field before going to the trouble of finding a Windows disc. Alternatively, don't use a password with the letter 'd', as it was the only key on the entire keyboard that suddenly decided it wasn't going to function anymore.