First thing I'd do is a repair permissions operation on the drive using Disk Utility.
Then, I would check the Sharing settings in System Preferences, make sure that you didn't enable any of the services like AFS, FTP, or Windows File Sharing.
If that doesn't resolve it, boot into safe mode (hold down the shift key at boot) and try rebooting there. If it works, there's some program running that is messing with things. So make sure that you close out all programs before starting the reboot process.
My wife's computer will not restart after downloading OS update. Download works fine, but when it comes up with the dialog to restart the system she clicks on restart and it tells her there is another active user. No other users are logged in. It does not accept her password to close anyway. She has admin privileges.