You probably have a worm. Try the easy fix at the end of point 2 in the thread below, it's worked for a lot of people.
WARNING: Viruses on the network you should be aware of

If you need more help, see point 5 for links to help sites. Norbiesworld is good & has helped a lot of LW users.

Please make sure that you're using a good AV program. Keep it updated & scan all downloaded files before you open them