Random CPU usage spikes?

For the past month basically ive just been getting the worst CPU usage spikes even on games that my computer shouldnt find graphically challenging. When i check device manager which ever game i am running just shoots up to 100% causes a massive lag spike / FPS drop for a few seconds then runs perfect again for 1-5 minutes then happens again. Its mostly on games where this happens because strangely enough i can stream HD quality video/movies/sports perfectly, no problems at all

Ive done a full virus scan and found nothing ive also tried disabling my AV whilst in-game and that has no affect either.

Any ideas what this could be and how to fix would be great...im also in a spot of trouble because i cant just try re-formatting as i have lost my windows disc.

My comp specs are -

AMD athlon 64x2 dual core 5600 2.91ghz
4 GB of RAM
500 GB HD
geforce 8600gt (1gb onboard ram)
Vista home basic 64



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Power supply failing, overheating? Open the case and check the fans for proper operation and make sure that any dust or other debris is removed