You probably have a worm.

Try the easy fix mentioned at the end of point 2 here, it's worked for a lot of people.
WARNING: Viruses on the network you should be aware of

If that doesn't help, there are links to help sites in point 5 of that thread.

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