Missus's computer has had some psu problems, first off sometimes the fan wouldn't start running and now it's rattling like it's catching on something. I know I can get a whole replacement unit for £26 off fleabay but do you think it would be possible to just replace the fan? If not with an original inside maybe an extra unit running off another plug (cd drive plug)?

have you removed the side cover and given it a clean down. There should be no reason why it catches unless the case had been knocked/ damaged, the bearing has gone or the fan blades have moved off the spiggot shaft

Ive replaced the 80mm fan in one of my PSU's. The power wires were soldered in on mine. easy job though. just make sure the capacitors on the board have discharged, as you could get a nasty zap from one if you catch it with a screwdriver

How shall I discharge the capacitors? I could always pull all the plugs so the motherboard and drives aren't connected (if those are the boards you mean)

Just leave it switched off but plugged into the mains for a while after you shut the 'puter down - but avoid touching anything on the circuit board if you can.

The fan bearings are usually the first things to go. You just need to get a similar airflow version but usually anything will do if you don't mind some extra noise. Tip - blast it out with an airgun before you start to dismantle it - these little buggers are like vacuum cleaners and there will be all manner of fluff & dust inside.