Make sure you have all your Windows, Program and Hardware CD's before even thinking about reformatting. Note some computers like Dell, Compaq, HP do not come with driver and Windows CDs.
Step 1 - backup all your data. You should do this regularly anyway. WARNING! Downloaded music liscences must be backed up before formating or your music is lost forever.
Step 2 - Gather the Windows CD, the Windows key and all your hardware driver floppies/disks. If you don't have the disks find/download/burn copies of the drivers before you reformat. Make sure all disks are working and you have all your keys. It is best to make a slipstream copy of your windows with the service packs pre-installed. thsi will save you hours. The alternative is to download and burn a copy of the service packs before reformatting. Install from CD is a lot faster and more relaible than trusting to a download.
Step 3 - Make sure your bios is set to Boot From CDROM first.
Step 4 - insert the windows CD and reboot.
-When asked if you want to set up or repair, hit Enter to set up Windows.
-Read the license agreement and hit F8 if you agree.
-If the setup program detects an existing installation of Windows and asks if you want to repair it, hit Esc to skip the repair.
-The setup program will ask you where to install Windows. Highlight the existing "C: Partition1" partition and hit D to delete it. Hit L to confirm.
-Next, hit Enter to install Windows on the "Unpartitioned Space".
-Format the space as NTFS and continue the installation.