You would need a copy of the Dell OEM XP install CD that was packaged with systems of that time. You could also purchase a new Windows 7 license and install it instead or go with one of the many free distributions of Linux.
Just use any recovery disc (OEM included). That has worked fine for me. I downgraded my Dell Vostro 410 to XP SP2 using a Mesh recovery disc with ease. SP3 is better as you can still update with that. I have been updating my (sic) XP installation only tonight.