TigerCat: If you have the original disk, just make sure your boot sequence boots from you DVD-ROM drive first

The first option you should get is Press Return to boot from DVD (or words to that effect) which of course you want to do. Then just follow the instructions. for setting up a new system.

Make sure you have the Product Key number though (25 digit code in 5 blocks of 5, numbers and letters) which will normally be on the install disk or the side of the machine, and like i said make sure you have anything you need to keep backed up first.

Its always best when problem get to much to start fresh

p.s. (edit) Do not worry about deleting anything, on install makes sure disk is formatted, which it will ask you, and all should be fine