If it's the fan on the motherboard, probably the bearing is going and it'll pack up eventually. They only clip on to the board so you should be able to get it off. Carefull as they're sometimes quite tight and need some pressure to unclip (a flat headed screwdriver is handy for this. Push down on the clip and then move it forward, it should pop off but try not to stab it through the board if you slip, thats not good, i know from experience). Once you've got it off you can take it to PC world or your local computer shop to get a replacement (they're not that expensive).
Hope that helps.
Chris, October 2005
