Unless something fundamental changed in Windows XP activation after SP1, then there is no reason that the Service Pack level would matter ... the product key does not care about architecture or Service Packs. The key is specific for Dell, so Dell media MUST be used for the key to work.  If you don't have it, you can try and order it (although Dell may not stock XP CD's any more), get it on eBay, or borrow from a friend - it just must match the version (Professional/Home).  There is also the possibility that you can activate it by phone with Microsoft.
I have an Otiplex GX520 which I am reinstalling the OS on.  The issue is that I don't have the original OS CD, so I used another Windows XP Pro SP3 disk for the install.  However, it will not take the OS Key that is on the computer.  The Key tag states it is for Windows XP Pro, so it should work.  I think the original was SP1.  Do I need to find an XP Pro Sp1 disk to make it work? I ordered a replacement CD  from Dell, but who knows when it will arrive and would like to get this resolved.  I am an MSDN member, so I can download any of the OS packages if I can figure out what the issue is. Greg
Reinstall won't accept OS Key