From what I remember when I did a full re-format before install the question read something like:
"this action may delete files needed by Windows to continue installation. Are you sure you wish to continue?"
Click yes and keep clicking yes until the format starts (gives about 3 warnings from what I remember) as you are doing a full format then full re-install and the files that are going to be deleted should be on the installation disk anyhow to be reinstalled when you get that far. It wasn't obvious to go down the route I eventually did, but when you get there you know you are where you want to be. I know how annoying all those different Users are, and you can't even access thier files, grrrrrrrrr!!!
PS I had a few extras to load on ie. soundcard data, graphics card data etc and it took around 6 hours from start to finish, hope you've got a good movie or two in the backgound
