So you want to go back to the way your computer first started, eh?
There's a few ways to do it, but every way requires the right tools. There are ways to format a computer for free, but
sending a computer back to the day you got it and formatting ARE NOT the same thing! Simply formatting is wiping EVERYTHING off. You will no longer have windows to use. What the discs do is wipe everything and then reinstall windows. If you do not have a method of reinstalling windows, then formatting will make your computer even more useless than you think it is now.
Some of the methods to format will require your serial/license/product key (I'll call it a Key).
The Key might be printed on a sticker on the side or back of your main computer machine (where you stick discs in), or it may be printed in the manual. If you cannot find it, you can run this program on the computer to see what it is.
http://magicaljellybean.com/keyfinder/
How do you know if you have a tool to reinstall windows? There's multiple ways.
1 - Use the discs that came with the computer.
If your computer came with a "restore" or "recovery" disc, put it in your computer and restart it. When you see something like "press whatever key to boot from CD" then press that key to start the CD. Follow the instructions to wipe everything and reinstall windows.
This is one of the easiest and most common methods. Either the disc will install the drivers you need, or there will be a drivers CD you use afterwards. Info on drivers will be at the end of this guide.
2 - Use a recovery partition.
Almost the same as the disc, except you access it by pressing a certain key combination when starting up your computer. This will install the drivers as well.
Not all computers can do this! Go to the site of the company that made the computer, find your computer model (It'll be on your computer's main body where you stick discs in) and get the instruction manual or info there, see if it tells you what key combination to press to activate it.
If the computer was wiped with some other method, then there's a possibility that the recovery partition was erased, so it might not work if somebody else tried doing it a different way first....
3 - Use an official windows install disc.
This is mostly the same as the first one, except that you will need to install the drivers yourself. If you do not have your own install disc, you can use soembody else's. If it's XP, you will need the same version of XP (like home or professional), but if it's Vista you can use any version disc.
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Your computer might need drivers in order for windows to use all the hardware of your computer as best it can. Without drivers, you will be missing things, like sound or wireless internet or 3D gaming. If you go to the site of the company that made your computer and find your computer model, they will offer all the drivers you need there. I suggest that you download them and burn them onto a CD before formatting, so you can install them right after you install windows.
You may not be able to use the internet without the right drivers, so PLEASE take the time to get them beforehand! 
