that the spare drive is 20Gb rather than Ghz?
If your computer was supplied with WindowsXP preinstalled and you didn't get a Windows CD with it you should have received a manufaturer's restore CD that will enable you to restore the computer to its factory state in an emergency. The Windows files will all be stored on a separate partition on your drive, and this is almost always a hidden partition.
Some time ago we obtained a ruling from Microsoft to the effect that it's permissable under the terms of the OEM EULA (End User Licence Agreement) to make a copy of this partition so that you could reinstall the Windows files in the event of a drive failure. You could therefore copy the partition to a seprate location if you wish.