|This is a "very common symptom" of a worn left click button switch on the mouse. Maybe by pure fluke you have two mice similarly worn - the left buttons do get a hammering.|
It would still be worth trying yet another mouse, before assuming it is a virus and fiddling around with virus checking software. If that's the only symptom then it sure doesn't sound much like a virus to me.
What happens if you push much more firmly on the left button while doing this? If it helps then the mouse itself is definitely the culprit.
How to know you are getting old 2:
Your feet seem further away from your hands