Another possibility, which just happened to me...I just got a Norton warning of a blocked attack. I was not on our site nor did I have a browser open. I checked the details and the file was in my "Windows restore" files. Norton reported that it had fixed it, however Norton cannot erase files in the restore folders so everytime you start the computer it is back.
I had to go into "control panel, system, and then choose the system restore tab." Check the "Turn off system restore on all drives." A pop up window will come up asking if you are sure and warning that all restore points will be erased. Click ok and your computer will erase all saved restore points. This may take a couple minutes. After that is done, click ok to exit. Reboot your computer and then go back into the system restore tab and uncheck the turn off box and click apply. When it is finished click ok to exit.
This seems to have fixed the issue on my system.