Has my computer got a virus?

[Qwertygiy]

My CPU Usage (as seen from my Task Manager) is always hovering between 50-75%, even when I have no windows open. When I have a memory-extensive program up, like Minecraft, Roblox, my music program, or multiple tabs in Firefox, it goes up to around 100%. Yet when I list the processes in Task Manager, there's nothing more than the occaisional 1% (besides the aforementioned programs, should they be running). I've also noticed that "System Processes" is always missing from the list when this happens. You know, that one that's always 100 minus everything else's CPU combined and not counted in the percentages?

Do I have a virus of some sort or is this just my computer being weird?