Usually the computer manufacturer will provide a method to be able to create a set of factory recovery/restore installation discs for this. What brand and model is your desktop or laptop? I'll be happy to see if I can post back with their instructions for how to do so.
The link from HP below will help show you how to create a set of recovery discs that can be used to reinstall Vista and restore the HDD the way it was when the computer came out of the box.