It's theoretically possible that your PC has a virus but it's far more likely that your mouse has senile dementia and will soon be on his way to that big mouse nest in the sky. (I've had the same problem on several occasions, as have friends of mine. Replacing the mouse has always solved the problem).
The good news is that it won't cost much to fix the problem. Tesco will sell you a wired optical mouse for �4.76. Spending 40p more (i.e. �5.16) will get you a wireless optical mouse. They're both just as good as far more expensive products.
Lol - thank you Chris.
Must admit the old mouse's looking a bit feeble these days, although it's not actually that ancient. Then again, time passes before you know it, so maybe I should get a new one. I'm using a wired optical Microsoft one atm, but wasn't sure whether the wireless ones were better or not. I'll check some out. Cheers.
Are you using a mousemat with lots of writing/patterns on it?
Optical mice are notorious for 'misbehaving' with these kinds of mousemats as they misinterpret the reflected patterns.
Try using it directly on your work surface (assuming it's plain!) and see if the problem goes away. If it does, all you then need to do is buy a plain mousemat.
Thank you Sputnik. The mat's plain - and no "worn" bits on it either. I've always used one, because the surface of the desk I work from is too good to risk getting even the slightest scratch on it - but thank you. It's something to bear in mind.