It depend on what version of Windows you have. If it's Vista, 7 or 8, you can reinstall WITHOUT reformating and WITHOUT loosing the data.
You need an install DVD of the correct version, any downloaded from some torrent site will do as it will detect that you already have a valid licence and use it.
Boot with the install DVD. You'll get the option "Update" or "Upgrade". Chose it.
This will replace/repair every files of the operating system but will NOT touch any user files, keep all installed applications and maintain all your settings.
Next, you need to let the auto-update download and apply all required OS updates.
If you have Windows XP, you need to save your data to some external storage before you do anything else.
For that, you may find a Linux install CD very usefull as it have a "Try before you install" feature that can be used to access your drive and copy the content to any external drive or burn it to some CD-R/DVD-R.
Once you have saved your data, you can proceede to reinstall Windows XP. It WILL suppress any data on the same drive as the OS, but any data on another partition or physical drive will be untouched.
This apply to any Windows desktop as well as any Windows laptop.
Electro, January 2013
