Happened to our 'old' (5 years) pc a few weeks ago. IT guy I got to know from a business network came round, just looked at it, said it might be the processor, took it away, called two hours later, had the processor replaced, brought it back and it's running like a treat agin
(Costs? £ 75.00! His alternative solution was to create a 'new' second hand CPU with working parts of our old one, estimated costs £ 150)