I just got rid of one yesterday. It was a trojan called Redirector something..... Everytime I clicked a link on any site it would install another one which my antivirus would catch and quarintine. But as soon as I tried to surf it would do it again and again. I finally tried empying my temp internet file and deleted my cookies in Internet explorer and then all was fine again. scanned the computer and everything was clear.
My wife and kids both go on Facebook and myspace often so that explains where it probably came from. I'm glad it was just didn't do any real damage.