i think a fan is gone on my PC its rattling its little bag off when i start it up... sometimes it overheats and shuts down meaning I have to pull the power lead out of it before it will turn back on at the switch..

there are a few fans on it when ive had the case off??

which one is it most likely to be and how do i go about changing them and where best to get them from?

 

sounds like the one on the processor (one on the main board)

hate to say it but pcworld might be the easiest .. deffo try a local computer shop first ..
normally screwed to the heatsink ... could be easier to get a complete new heatsink and fan..