Reinstall product key

My wife's custom died [mother board].
I got a dell optiplex GX1 with no hard drive
Installed the HDD maxtor 40g [Diamond max] & told bios to boot from disc.
I got stuck on the product registry key screen
I have a windows xp sp1 disc & the registry key form the custom machine.
I get an error [invalid number].
how do I proceed?
I can only get to continue install or recovery...
At recovery I only get to choose
C:/WINDOWS>
I either need to discover the dell product code or reset so I can use the code that matches the disc.
When I try to follow the procedure for discovering the key as if I were at the dos prompt, I get an invalid command error.
I don't need any of the data on the HDD, just a functioning xp pc.