Whilst it can be numerous things, it most likely is down to a virus.
My general advise for similar situations:
Start with a good tool such as Spybot Search and Destroy or Malwarebytes Anti Malware and perform a full scan.
After this has finished, use Microsoft / Sysinternals Autoruns and look through most of the tabs (you especially want to pay close attention to Logon
and Scheduled tasks
) and delete most of the items that you think are viruses or research them first.
Next, protect your system with a good antivirus. I personally recommend Microsoft Security Essentials.