Just rebuilding an old PC I picked up a few weeks ago and windows 7 is running fine on an SSD. It will restart out of the bios, click off and boot fine. On shutdown the screen goes blank and all the lights go off, but the CPU fan won't stop however long it's left for. A 4sec push of the power button stops it fine. What is amiss? It was fine when I bought it and has started behaving like that all of a sudden.