Seems to me a like a simple ClassicShell/Explorer crash which can be caused by many things. If you terminate Explorer and/or ClassicShell in the Task Manager and then restart Explorer (File>Run new task>Type in "explorer" without quotation marks>OK), it should clear the problem easily.
I notice that this stuff commonly happens to me whenever a game takes focus and doesn't close out properly, and Explorer panics a bit.
Try doing virus scans with MalwareBytes (which is free)
This will literally never be the result of a virus. No virus is designed to simply bug out your explorer bar. Anybody who thinks that is, frankly, a colossal idiot.