Sounds like a virus. Have a look at the sticky below
WARNING: Viruses on the network you should be aware of

Try the easy fix mentioned at the end of point 2, this has fixed the problem for quite a few people.

Otherwise, see point 5 for links to help sites. Norbiesworld has helped a lot of LW users.

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