Have you simply tried hitting enter when it asks for a password?
If you did or that doesn't work, repair install is best. There are ways to set a password with Linux-based Live disks, but that could be more troublesome then it's worth. Repair Install
Very important for security and sharing ability reasons to always set passwords on administrator accounts. If you want Windows to boot up to desktop still without you entering the password, use netplwiz
in the start menu search box.