Thanks Sky 330
Your reply was much appreciated......... thanks.
The hard disk was VERY quite and no "chuntering" from inside the PC.
This is the first time it's ever happened, but perhaps it was the latest software I installed was the problem........... SKYPE, the free internet phone system.
I had it "off" when the problem started (the mouse slowing up) - or maybe it's not "off" when I thought it was!
The PC seems to be running fine now, but what a strange glitch.
Cheers
TG