Don.Christie
If you reside in the US and are the registered owner of the computer, a disc containing the factory-installed operating system specific to your system, can be requested from HERE.
Note: Dell may make a charge for the disc.
If you don't reside in the US, contact Dell's Support that's located in your country.
I've also seen Dell reinstallation discs for sale on eBay.
No, a Dell Vista reinstallation disc cannot be used on a Dell system when XP was factory installed.
Bev.