Today, Linux saved the day... again.
My dad called me to ask me what to do when his Windows computer didn't let him log in.
He has a single account and a blank password.
I searched the internet and found a post explaining a great little utility that allows to reset passwords, unlock accounts, etc.
The command is chntpw
The instructions in the following link use the Linux rescue CD but you can easily use your favourite Live CD and install the utility from the repositories.
My dad has a dual boot computer so I've already installed the utility for the next time he locks his account.