If you have a regular Vista install disc, you can tell it to install Windows on the same drive, but keep the old OS. This will repair your master boot record and keep your files in windows.old along with the program files and the old user accounts. This starts you over, but keeps your data.
If you have a Dell restore disc, it will want to wipe the drive and bring it back to factory specs. There may have been a hidden partition that got wiped that it would have used to restore but couldn't find and failed.
A third option is to mount the drive on another system and repair or back it up. You would need an external enclosure.
Have you called Dell?