Im not positive you can actually format the drive and install Vista Ultimate if this is an upgrade disc.You may have to install Vista Home first.
Firstly,do you have the Vista home disc ?
Secondly,do you still have the upgrade disc ?
Thirdly,do you have the motherboard drivers and utility disc ?
If so,you maybe able to format using Dban or killdisc but your Vista Home disc should do this for you.
Have you checked for a recovery/restore partition as Dell machines normally have this.This will revert your computer back to factory settings.