If you have a CD, you have 2 choices: 1: Put the cd in, boot from the cd, reformat everything, reinstall from scratch. You might want to backup your stuff before doing that though, cause that will wipe everything. 2: Put the cd in, the autorun will ask you if you what you want to do, ask it to reinstall it, and it will reinstall it. However, it won't reformat everything, so it won't be a super clean install... If you don't have a CD, you can find/copy one from someone else... Copying the cd is perfectly legal, since you (I suppose) have a legal activation key already. Make sure you know the activation key of your installation, though. If it came with your computer, there should be a sticker somewhere on your computer case with the activation key. Also, make sure the CD you have is for the same version as your activation key. A key for XP Home edition will not work for an installation of XP Professionnal. But as I said, reinstalling windows will wipe out everything you have... If you had your computer for a relatively long time, and never reformatted, it's probably a good idea. Your computer will once again run as fast as when you first used it. Don't reformat just to be able to use DCME lol But it's usually good to make a full cleanup every few years. To backup your files, just burn them on cd's/dvd's. Note that it is completly useless to backup programs. You will need to reinstall programs, cause just copying them back would not work. Anyway... If you never done that, I suggest you do this with someone who knows how to do it. Not because it,s complicated... but just to make sure you don't end up not being able to use your computer at all And if you have enough hard drive space, you could create a second par!@#$%^&*ion (the Windows installation allows you to do that easily). You would put all your data files on that par!@#$%^&*ion (movies, music, stuff), and the next time you need to reformat, you wouldn't have to touch that par!@#$%^&*ion. You could even just make a backup of your files on it. You'd only need to wipe the Windows par!@#$%^&*ion to reinstall it. You can find a lot of guides on Google to do all the things I talked about...