Set on the gas more!! (yuk-yuk-yuk!!)
There are many different things which can slow a computer down. YOu didn't say how old it is or what you have on it or how you use the computer.
If you're using your computer for gaming, you may need a better newer one that can handle gaming better.
YOur Windows might be older, so it can't run the computer as fast.
You mgiht not have enough RAM, which is used to help process stuff. If you have more RAM, the cmputer can process more and faster.
Having a large enough hard drive also helps, because if it's so full, it's harder to place new info and retrieve it.
Your chip might be too old, might be too slow to handle all you ask of it. For anything modern, you should have at least 2Gig chip speed. And at least Windows XP.
The other obvious is that it's infected with virus or cookies. They can mess things up easily. Goto MajorGeeks.com and you can get some excellent FREE antispyware programs.
Also download some cleaners such as CCleaner and Eusing Free Registry cleaner. These get rid of unused or old folders in your registry. It's like cleaning your bedroom so you can walk around.
Then also download Auslogic Disk Defrag, which rearranges stuff on your hard drive. Kinda like putting stuff in the closet, in the drawers, under the bed, etc. Once you use this, you only have to use it once a month or less. But the registry cleaners should be used regularly, like weekly, along with some good antispyware programs.
I had a friend who had such a bad infestation of cookies, the computer would freeze up immediately. It took over 2 hours to clean it up.
Get antispyware progarms from MajorGeeks such as Spybot or SUPERantispyware. These are excellent for keeping viruses off your computer. But these also need to be upgraded regularly, like weekly or every couple weeks, to make sure they have the up to date info to work with.
Also download SPYWARE BLASTER, which works kinda like a fence or firewall to help keep stuff from getting into your computer.
If it's spyware, these will definitely clean you up.
But as you can see, trying to speed up a computer is about like trying to speed up a car, there's too many things which need to be done.