I like being in control of my computer. I don't want anything taking up my processor power unnecisarally. So when I notice something installed that I never asked for or wanted, it makes me so angry that I could eat a gallon of ice cream (isn't that what you do when you're angry?).
Spyware is any program that uses your computer without your consent. The main two things that spyware will do is try to show you advertisements and try to steal your personal information. They do other things too, but the bottom line is, you don't want them. I would say, the number one reason that computers slow down over time, is because they are bogged down by spyware.
Removing them can the tricky. After you spot the offending program, many times a simple uninstall will not do the trick. Fortunately there are a few programs that can help you out. My personal recommendation is, Adaware. It's free, it's easy and does the job. It's a good idea to keep one anti-spyware program installed on your computer at all times. I also recommend installing a second program every few months or so and find what your main program might have missed. Other programs I recommend are, spybot (free), and spysweeper (30 day trial for free, so it's good for a one time check).

Download Adaware!

If you have never done a spyware scan before, it will tell you that it found 4,304,392 (well a huge number anyway) items. 95% of these are not that important. There may only be two or three critical items. They just like you too see a big number to make their product seem more important.