Windows won't shutdown

I've been having a problem with shutting down Windows (Vista Ultimate). I
don't know when it started nor can I trace it to any specific program or
setting but Windows refuses to shut down properly. It usually hangs on the
shutting down spash screen for several minutes and then the computer
eventually reboots itself and goes back to the user log-in screen. I've also
noticed that it continually attempts to check the disk after rebooting but
aborts and states that the volume is locked and then proceeds to the log-in
screen. I'm also having parallel problems with loading my McAfee Internety
Security Center as well. I don't know if any (or all) of these problems are
related somehow but I can't seem to figure out what could be causing them.
Any advice would be greatly appreciated. Thanks!