Sounds like you might've downloaded a nasty. Maybe you should do an AV scan, also scan for adware & spyware

Check the links below for reliable, free programs. AVG Free, Ad-Aware & Spybot are good ones to start with!
http://www.gnutellaforums.com/264170-post2.html

This thread is worth a read, too.
WARNING: Viruses on the network you should be aware of