I think the concern is most likely a roommate. If this is a workplace, I'd say, go talk to your supervisor, and get approval by your IT people AND by your security people. (And, often, a workplace can require you to lock your computer to prevent this.) I suppose it could be a workplace, if you are the boss.....at that point, what you're asking makes perfect sense. I'll go further. Don't respond to these surmises. Doing so can't do any good, and COULD hurt.
I suggest you look here:
http
/monitoring-software-review.toptenreviews.com/
Found just searching on "pc access monitoring software."