How to Reinstall Windows Without Losing Your Data

"Over time, Windows loses stability. If you keep a computer for more than two years, at some point you're going to have to bite the bullet and reinstall Windows from scratch. But contrary to popular belief, you won't have to reformat your hard drive (with one exception, discussed below). The bad stuff you need to get rid of is all in your Windows folder.

Before you begin, gather your Windows and application CD-ROMs. Back up your data files (just to be safe), and then clear two days off your calendar. If everything goes smoothly, you can reinstall Windows in a few hours. But you have to assume something will go wrong: You may not be able to find a necessary CD, or data won't be where you thought it was, or something will simply refuse to work." (more at link)