Firefox, Spybot, and Ad-Aware as already suggested. THey are great. I also tried Microsofts new antispyware program. It found 2 programs that Spybot and Ad-Aware did not. I also love AVG antivirus. free download from www.grisoft.com
There are many things you can try, all depends on the OS you are running. http://www.analogx.com/contents/download/network/pow.htm I run this on all of my preXP SP2/2K3 Server machines. Works great without the overhead of other programs. Becareful of some of the "adware" and "spyware" removers, they either contain spyware or trojans. I have had good luck with adaware from lavasoft.de and spybot s&d.