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

Thnx!

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