One other dreaded thought is that the hard drive might be failing.
Mine went like that in the early stages.
Ended up having both replaced under warranty, 20 months old.

Oh my! That's about the age of mine, which is a Dell. What was yours?

Did you try a different mouse? Perhaps it is having issues.
It doesn't really sound like anything else to me. Hard drives usually dont give much warning before they fail - they just do. In my years of computer nerding, I've probably had about 4 drives fail on me. None of them gave warning - they just went dead. Same with the 2 fried motherboards I've had.

As far as viruses/malware, take this advice from a total computer nerd: Get a free antivirus called Avast. I have used it for years and it has caught everything without ever once recognizing something as a virus that was not. Look up reviews of it if you'd like - it's considered one of the best, if not the best, out there - free or otherwise.