Most PC's these days have some sort of Factory restore, this is the best way to restore the PC.
Installing Vista from DVD will destroy this factory restore option forever, be careful what you do.
Most Dell PC's do have a factory restore, you can reach it by tapping F8 during the Dell bios splash screen, this should bring up more options including the restore feature.
http://support.dell.com/support/topi...7D4&doclang=en
"will the vista cd require a product key to install?" It should be a DVD, and if it is a Dell Vista DVD, No you don't need a product Key, it will install without one and activate itself.
Request a DVD from here
http://support.dell.com/support/topi...dhs&redirect=1