I installed a new big HDD the other day, and since then my mouse (wired) has gone on the blink, it wont work for a while then it does, sometimes I unplug it and plug it back in for a while (USB) and it seems fine, then it plays up again.

Sounds like a broken wire in the mouse cable, certainly if it's plugged into a motherboard socket on the back.

If it's on a socket that's cabled to the motherboard (one of the front panel sockets) you may have disturbed that cable when putting in the new HDD

First thing to try is plugging it into a different USB socket. If you see the same behaviour try another mouse.

Cheers Ian - I'll give it a go.

Ok, so I've swapped mouses, swapped USB ports to a completely separate pair, opened up the PC, checked all the connections, air dusted the whole thing (very dusty!!) and it's still doing the same thing, which leads me to believe it's a software problem. So any clues about how to deal with it?