That could be legit. Windows runs things when idle like indexing, scanning for malware, updating etc. As soon as you move your mouse it reduces cpu usage. Which technically looks like a malware trying to run in background without user knowing it.
At some point, I got some mild paranoia because every time I opened the task manager to discover WTF was eating all my disk bandwidth, the usage suddenly felt from 100% (with a queue) to 3% to 5%.