There is no "magic" solution for speeding up a computer.
Some web sites may offer to speed up your computer by clearing out the registry or deleting unwanted files, but anything like that will make a minor difference, and if they delete the wrong thing it could stop your computer woking.
Has it always beens slow, or just started happening?
What are the hardware specs of your PC - CPU, memory, hard disk?
Did it have XP on it when you bought it or have you upgraded to XP from an earlier release of Windows ?
I think the only REAL way to speed up your computer is to completely re-install Windows. Thats gets rid of all the rubbish that gathers during the years, but this may be it bit drastic for most people.