Hello there, It was running XP Home at the time of trouble.
The fan was serviceable at the time, but I guess it's possible it may have stopped working this morning.
The PC was working fine earlier today (on for a few hours), then it reset itself. It then reloaded and reset itself again over then times. The time frame between each reset got smaller each time.
Now that you have suggested it, I will invest in a new fan in any case!
Thanks!