Was a drink spilled on the keyboard at all, ever? Perhaps there is something stuck under the key that when pressed, isn't pressing down the whole way.
Definitely not a virus, it is possible it's the keyboard itself. How old is the keyboard? Get a can of compressed air and spray out the keyboard. If my keyboard had this problem, here is what I would do:
I would turn off the PC first, unplug and spray the keyboard with the can of compressed air, check to make sure USB's are clear of any debris, plug keyboard back in, turn PC on. If it does it again, get a new keyboard. Also you can try to pop the key off the keyboard and check under the key if it is still doing it after cleaning and etc.
My own personal computer experience of 25+ years.