Look for network issues or roaming profiles. I know there were a few bugs where it would try to load (or save) roaming profiles and fail, causing a very slow boot (or shutdown).

Otherwise, check to see if your domain admin folks are dumping large amounts of "updates" or "pushes" to your system. That is something I see a lot of here.

Lastly, make sure you have a local machine admin account. Try logging in with that, shutting down, restarting and use this account to try to diagnose. If it is getting hung up on a network or domain issue, this might help clear it.