Is it the release candidate of Windows 7? Because that just expired at the beginning of the month. If I remember correctly it will continue to shut down after a few hours until August when it just won't go on, at all.
I was having the exact same symptoms with the release candidate until I reinstalled Vista a few days ago. It fits the symptoms, but I would imagine you'd already know this as the release candidate gave plenty of warning that it was going to expire for the entire last month.
