You may think that your computer is virus free but...
Just in case Download and try a different virus scanner like AVG free version.
Also make sure your Virus signature files are up to date.
The only thing that would cause a problem like this is either a virus or trojan file.
Perhaps a video or music file you downloaded contained a little surprise.
That's why it's good to always scan every file you download the minute you download it.
Most Av programs let you right click and scan the file as soon as you have downloaded it.
I would download the free version of AVG Antivirus and see what it can find before you go much further. Not all AV programs are equal and that goes especially if you haven't been keeping the virus definition files up to date. New viruses or dirivitives of existing ones pop-up nearly every day so it's really important to be sure that your virus scanner keeps it's signature files up to date.