yeah, but what about the HD?
I just think byteguy is right on with this:
Perhaps you should look at the task manager and find out what is running. Chances are something's hanging up and preventing the system from shutting down.
That happens a lot of the time with people that i service. Weird ass apps that are running and need to be killed off in either the registry, msconfig, startup or whatever. If its not this (and since u reinstalled windows again and it still doesn't work) you should try using a new/known-working HD instead.