"When it had a go-slow moment, which it often does, you can hear the HD or something in the PC working hard."
The ONLY thing you're going to be able to hear is the hard disk. It's possible that in some circumstances a fan might get louder if the PC is being used hard (gaming) but you're not doing much with the PC, so the only thing that's going to make a noise during "go-slow moments" is going to be the hard disk. Providing you don't have any issues with your CD drive etc. But I'm sure you'd have mentioned if it only happened when you opened an explorer window or something.

But I think the advice so far has been good: a good new install of Windows is always nice. And it's less hassle than you imagine it. The amount of times I've ended up having to reinstall, and it's so much less aggro than you imagine it.