That is another bit of information that would convince me it's a virus.  If it is, and the system is infected, the system API and processes are most likely hooked by the virus so Virus protection and scans run from the infected system may not work properly, and will not find anything - that is why I say it is so essential to boot from a clean virus recovery device (USB or CD - I prefer USB as they can be updated once booted).  Some of them may even seem fixed for a time when doing reboots and updates, but will flare up again later.

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