XP - how can I get my licencing code

Slightly long convoluted tale here but bear with me. I bought a Vostro 200 a couple of years ago. It was being sold as a Vista machine but with an option to have XP loaded onto it instead. I paid extra to have the OS downgraded to XP from Vista. I got a Vista CD and the Windows label stuck on the computer has a Vista serial number on it. I don't have any data relating to XP Professional which was loaded onto the computer by Dell. 

Roll on to this weekend. I decided to reinstall XP and because for some reason I couldn't access the recovery partition (which is still there, incidentally) on the computer, I borrowed a friend's XP CD and began to reinstall XP from that. It was only when the time came to type in the serial number that I realised to my horror that the code stuck to the computer was for Vista, not XP.

Is there any way I can find out what my XP licencing code is? I never got any piece of paper from Dell relating to this so I'm high and dry. I assume because I bought the PC from them that they'll have it on record?