Try opening your case and wipping away all the dust (on the cards and on the fans, unplug the power cord from the wall before doing it, and discharge any static you might have on you by touching the case before anything else). Try to locate where the CPU fan is connected to the motherboard, then unplug it and wipe the dust off around that place. Then replug your fan again. If you're lucky, it will work. If not, get a new fan.