Joe keeps getting an annoying popup, and he has no idea where it's coming from. Leo says that popups usually come from installing a tool bar. Often, installing shareware will come with an additional "payload" because users didn't uncheck and disable it from installing. Leo calls it "sneakyware", because it slips by the user when they're installing something.
Joe can uninstall this in his control panel under "Add/Remove Programs". He should also look for an uninstaller in his programs. That chatroom says that CCleaner will uninstall any toolbar or sneakyware that's installed. Revo is another option for this. Sadly though, it could be even more nefarious. If that's the case, then the only real way to get rid of it is to backup his data and reinstall Windows from a known, good source. Then run all updates.