I have the same problem as the person who posted before, with this wrinkle: I have tried the onscreen keyboard to login, and it doesn't respond. The only buttons on the screen that are responsive are the shutdown and login for the default administrator "user" account. I cannot get into my main account because Vista no longer registers keystrokes in the login shell. I have tried a different keyboard without luck, though it is apparent that my original keyboard works fine anyway because that is what I am using now to type this from the "user" account, which didn't have a password (luckily) or I'd be locked out of the system.
I tried a system restore to a few days ago, but it didn't help at all.