the XP license on your Dell is OEM and from MSDN it is retail. The keys aren't interchangeable. You need an OEM version of XP MCE or better yet a Dell XP MCE 2005 Reinstallation DVD which will automatically activate within the Dell BIOS without input of a product key.
Hello! I try to reinstall Win XP MCE on my DELL XPS 710. I downloaded Win XP MCE 2005 CDs from Microsoft MSDN Software Repository. The license sticker on my DELL XPS is in a good shape, so I can read all the characters. But the Windows install routine tells me, that the key is invalid. Could one assist with that issue.  Thanks. Tom
DELL XPS 710 - Windows XP MCE Key does not work