My computer was fast when I bought it, and now it seems slow. Why does this happen?

This effect is really a combination of things:

When your computer was new, it was faster than most of the other computers that you may have used at the time. Now, it is slower than the other computers that are around. The computer seems slower, because your frame of reference is different.
As you use your computer, you will often download programs such as realplayer, aol, antivirus software, and other programs that run whenever your computer boots up. These programs usually show up in the system tray by the clock. Each of these programs takes up system resources (memory) while it is running. This will cause your computer to slow down.
As your computer is used, the hard disk drive becomes fragmented, which slows down the amount of time it takes to read a file off the hard drive. This affects the speed at which programs load as well.
You could have spyware. Spyware is a general classification for software that is designed to track the way you use your computer. Spyware usually just tracks your internet browsing habits, and sends you popup ads based on the sites that you regularly visit. Some spyware is much more insidious, and causes you to receive unsolicited emails (SPAM), or even places pornography on your machine. A competent computer professional can assist you in removing the spyware from your computer, and immunizing your computer so that you are less likely to get spyware again in the future.
Resources:

Spybot
- http://www.safer-networking.org/

Ad-Aware
- http://www.lavasoftusa.com/