We need some more information about your computer to tell you what exactly is causing the slow down.
So it will be nice if you coul tell us what the configuration is and what you use the computer for. I mean just to surf the net or running any other specific software/applications etc.

All the solutions here could help you but then again it could be a differnet problem all together.

I dream of a better tomorrow where Chickens can cross the Road without having their motives questioned - Unknown