Realy easy techy...boot up from the Vista DVD. Instead of selecting "Install" Select "Repair" and ir comes up with a load of options, one of which is a command prompt. go to this and you'll get an old style Command prompt.
Type
then press enter.
Type
the press enter.
This will list all of the volumes on your hard disk and and DVD/ CD rom drives.
Assuming you only have one hard disk volume, and lets say its volume zero in the list...
next type
(replace 0 with the number of the Hard Disk Volume shown in the list command)
then type
This will destroy the volume on the hard disk. You can then reboot from the Vista DVD and it will see it as a completely blank disk.
NOTE!!!! You should be careful if doing this on a PC that came with Vista pre loaded or has a recovery partition as you could end up wiping it! Only do this if you have the MS Vista DVD and a proper license key and you realise you'll have to reinstall everything again.
If you have more than one Hard Disk volume shown when using the list command, post the list here and I'll try and work out which is which ;-)