Users all over the Net (including a few members of ExtremeTechs own staff) are hopping mad about a nasty bit of spyware called Xupiter.
Xupiter is not only spyware but also "foistware" -- that is, it installs itself on many users machines without asking permission. Xupiter "infects" Microsoft Internet Exploder via the browsers automatic ActiveX installation feature when you visit certain Web sites. (Due to lax default security settings in many versions of MSIE, most users never get a chance to say "no" to the installation; it just happens.) Users report that once this "drive-by download" has taken place, the Xupiter toolbar hijacks their browsers. It adds unwanted bookmarks to their "Favorites" list, launches annoying "pop-under" ads, displays an unwanted toolbar in the browser window, and generally interferes with the users work (not exactly the best way to win friends and influence people). It also sends information on users browsing habits to an advertising server.
Detailed technical information about Xupiter can be found on Alan Lubers "official" Xupiter page. This page explains how to find out if Xupiter has infected your machine, and how to secure Internet Explorer so that it wont happen again.