You could have a look in Event Viewer to see if that has reported what could be causing this, but if this was my computer I would restore it to Vista, either by a full system image I'd created before the upgrade or using the factory reset - run the Win 7 Upgrade Advisor - as in your case as I would have already have run it and if that gives the green light, redo the Win 7 Install using the Upgrade option, using the steps as in the tutorial I have already linked.