Hah!
How strange. I've sorted it out until the next time my case clogs up
with dust and crap. Heh, such a strange problem.

I decided to open it up, and get a can of pressurised air in there
So much dust came out it was unbelievable. I also fiddled with the CPU
fan a little, and tucked away any outstanding wires. And now, as far as
I know, it's running fine!
What can I say?
It's great ot be running fullspeed again. My PC wasn't even this fast
when we bought it
Thanks for all the help guys. This'll be something I bookmark in case I
ever have problems in the future