Sounds like a virus. Info & advice in these threads
http://www.gnutellaforums.com/showth...threadid=39612 http://www.gnutellaforums.com/showth...threadid=41432
The bottom thread has a link to norbiesworld, he can help you get rid of the virus if you're having problems.
You need to make sure that you have a good, updated AV program & that you scan files before you open them