First, follow the steps outlined in this thread:
http://www.daniweb.com/techtalkforums/thread5690.html
I'm pretty sure you will need to post a hijackthis log (explained in the thread). If you do, please post it in a new thread in the Security forum.
Also, empty the contents of all "temp" and 'temporary internet" folders for all users. You should also do a search for *.tmp and delete everything that is found.