Well, Indeed over the years your system will clog up with stuff, and us techie type people like to reformat and reinstall windows quite often, to have a clean running system. So there would definatley be an improvement in your computer. If you do this, it will wipe your hard drive clean, empty, blank. So anything you have on your computer, you will lose, unless you back it up to CDR or Floppy. However if you have never done this before, it can be a difficult task. And if problems occur, you'll be stuck. So... do you have a friend who knows computers? If so you might want to ask him if he would help you do this. Its much easier to get help in person.
If you want to do it yourself, you will have to follow alot of written instruction. Also if you have any problems, you wont have the net to get onto!