The Dell Windows 7 Reinstallation DVD will use system locked preinstallation inputting a Dell generic system locked preinstallation key which will automatically activate if and only a valid Dell BIOS is detected. Likewise for the XP Reinstallation CD. The COA affixed to the system should be Windows 7 Professional and should have a unique 25 digit product key for Windows 7 Professional. This does not need to be input as system locked preinstallation is used. See my Windows Reinstallation Guide for more details: [URL]  
I purchased a precision 7500 workstation in 2010 that came with win 7 but I asked it to be downgraded to xp. I am ready to install the win 7 OS from cd that came with my original system but am unsure if I need a new product key or can I use the same product key that was activated as an xp OS for the win 7 OS?
can I use the same product key for a downgraded win 7 that I am ready to move 
from xp to win 7