Why does a blue screen keep poping up and restarting my computer?

I have a windows vista that I've had for the about three years,a blue screen with white writing pops up and then it restarts and says it has been shut down in correctly it's been doing this for the last month or so but all tests show that it's virus free and running normally but Mcafee real time scanning has also been turning it's self off, when u turn it on it turns back off but then u can run scans or updates and it will stay on