You could check that you have fast user switching enabled. Go to Control Panel>User Accounts > click on change the way you log on or off and note if the box for enable fast user switching is checked. If not do so then go to Control Panel > Admin Tools > Services. In Services scroll down to fast user switching. Double click this line. There are three options. Automatic. Manual and Disabled. Set this to Automatic. See if it makes any difference to the time it takes you to shutdown.