Thanks both, have checked with the owner, and it seems that she has recently bought a new printer which has been positioned so that it partly blocks one of the PC casing vents. I've told her to move it and make sure the PC is properly ventilated, and not near any heat sources (like a radiator, for example).
If this sorts the issue out, I owe you both an equal share of the Mars Bar she has offered me as a reward.