Ad-Aware 4.0 is available at
http://www.lavasoft.deA nice little freeprog for zapping pop-ups like the one this caused--it shuts down any window with a title kept on a list, like "Visit our store!" or "Celebrity Buttshots!"--is available from
http://www.jpsoft.dk(Bear in mind, these are just two of many such progs. You can get a comparitive list of free and shareware progs of any desired variety, with user reviews, links, and whatnot at sites like pcworld.com, zdnet.com/pcmag, and download.cnet.com)