Well, uh, let me explain.
If you are using the same computer you have used for more than two years, then normally one should notice that their computer does not run as fast as it did the day they bought it.
Reformatting completely erases the entire hard drive, removing all the useless crap from your computer. I back up all my important stuff (HL, CS, WMP, divx, aim, etc) to disk, and format. When done, I have a nice, clean slate to start off with.
Somehow I think 1 out of every 10 songs you warez has some kind of virus or lagger or something.
I know it is time to reformat if I click on start and it takes more than 1/2 a second to bring up the start menu.