How can I find or Get a C.O.A. for a Dell-Windows XP ?

I have a Dell computer running Windows XP professional that was a refurbished computer from my school. The computer no longer has a sticker that has the COA, and is not connected to the internet. I know the: Serial #, Model#, Mfg Date, PID, and the version of the OS. How can i get a new COA or find out what this one is?