One either came with your computer or can be burned from a recovery partition on your PC. Consult your OEM's documentation for your model in order to find out how to boot from it to do a recovery/burn a set of recovery discs. If theses are not available to you, you will need to contact your OEM for a set for a small fee. In the meantime, you can also re-install Vista with a friend's DVD using the product key on the sticker attached to your computer, though it will not contain any pre-installed programs your used to that the recovery set may have.