You have a virus. Probably from downloading programs & not scanning the files with your AV before opening them. Program files are often infected. See the sticky below
WARNING: Viruses on the network you should be aware of

Try the easy fix mentioned at the end of point 2, it's worked for a lot of people. Otherwise, see point 5 for links to help sites. Norbiesworld is a good one & has helped a lot of LW users.

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