PC Magazine has a review roundup of personal anti-spyware tools for Windows. If you just want the results skip to the Editor's Choice section. I personally use Lavasoft's Ad-Aware on my work machine. If you don't run anti-spyware I'd strongly recommend you download one ASAP. It's unfortunately a necessity in this day and age.
Also, ZDNet has a lengthy review of the top anti-spyware tools for corporate/small business environments.
Some common symptoms of a computer infected by spyware are: random popup ads, computer slowdown, computer crashes or the browser homepage changing mysteriously. If you are experiencing any of these then it may be possible your computer is infected and you should run an anti-spyware application and hopefully it can remove it. Good luck.