My PC had been switched on for about 12 hours before it began. It continued even when all programs and IE were shut down. It stopped beeping after about an hour. I've noticed that there is now an interimitant soft buzzing noise coming from my machine when switched on. Could this perhaps be the fan, which would tie in with your over-heating suggestion I suppose?

If so, can you suggest the best remedy for overheating? a new fan? a spring clean inside? (the fan outlet is looking a bit dusty...)

Thanks for any help you can offer.