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
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.