FYI: even if you have the latest antivirus software, you still aren't safe. Symantec Antivirus Corporate Edition 9.0 does have a spy/mal/adware detection feature but it isn't done as extensively as the programs stated above (not to mention the 3 programs above are highly recommended by security sites and other reputable sites like cnet.com / zdnet / etc.)
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from cnet.com
#1 Ad-Aware SE Personal Edition popular
#2 Spybot - Search & Destroy popular
#22 22 SpywareBlaster popular
http://www.download.com/3101-2001-0-1.html?tag=pop