During my Computer Safety and Security Seminar, I often mention the fact that Windows machines tend to slow down over time, and the best way to cure this is to rebuild the machine. It's not a small job, nor one to be considered by anyone who isn't somewhat computer literate. It'll also take at least 2-3 hours, sometimes a lot more depending upon your internet connection speed or if you encounter any problems. You should only really attempt this if you have another computer, or as a last resort on an old computer, as if anything goes wrong, you'll be left with an inoperative machine. Finding a friend who has done it before to help you is definitely a good idea.
The first and most important step is to back up all your data onto an external back up drive such as the TechnoRV Executive Style Backup Drive. 160Gb should be ample for most older laptops although 320Gb is worth the extra. After that, you'll need the Windows XP installation disks unless your computer has a seperate partition containing a copy of the operating system, and an internet connection. If you don't have the Windows disks, try contacting the computer manufacturer or looking for a cheap replacement set on E-Bay (make sure to buy the disks for your exact machine, otherwise you won't have the correct drivers etc).
To actually perform the re-install I've provided a link to an excellent article on WikiHow. It covers pretty much everything you'll need to do, although you probably won't need to install all the software they recommend, think of that as more of a reference.
Best of luck, and remember, this operation will destroy everything on your computer, so make doubly sure you have everything you need backup up, and preferably copied to a different machine so you can verify it.