This is usually caused by a worm.
Try the easy fix mentioned at the end of point 2 in this thread, it's worked for a lot of people.
WARNING: Viruses on the network you should be aware of

If that doesn't fix it, I'd suggest you go to Norbiesworld & get him to check your computer out. There's a link to the site in point 5 of that thread.

Always make sure that you're using a good AV program. Keep it updated & use it to scan all files that you download before you open them.