Sounds like you have a virus. See point 2(h) here
WARNING: Viruses on the network you should be aware of

You should go to Norbiesworld. Go to the site, register & follow the directions for free computer help. Norbie will be able to help you get rid of the virus.
Make sure that you're using a good AV program. Keep it updated & scan every downloaded file before opening!!

When downloading video, check out the file before download with Bitzi
How to use Bitzi