Sounds like a virus. Go through the thread below, try the easy fix mentioned in point 2...that'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 help sites. Give Norbiesworld a try, it's free.

Please make sure that you use a good AV program. Keep it updated & scan downloaded files with the AV before opening