Well, this is what I do to figure out the reason for eternal shutdowns. Go to task manager (ctrl+alt+del popup-> task manager) and look at what you have running. I usually shut one thing down and then try to reboot. If it still takes forever, then I shut down something else, and so on until I find the culprit. I have found that most Symantec products (namely WinFax Pro) cause my shutdown/reboot hangups. Good Luck.
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Regards,
clutch