The Product Key is the 25-character code on the sticker on the computer case.  Keys for Home Edition are not compatible with other editions (Pro, Media Center, etc.)
I purchased this Dell Dimension 2400 desktop in 2003 as a wedding gift for my daughter and son-in-law.  They are busy and didn't use it a lot, but it just got to the point where it needed the OP reinstalled.  I have done this before on my computers so I offer to do it for them.  When my daughter went in to retrieve the disks that came with the PC there was no windows xp home edition op reinstallation disk included and I don't think I ever got one with the PC.  The other cd's were there.  I decided to try using the windows xp home edition op reinstallation disk from my Dell Dimension 2200.  I was told it was okay to do that when I spoke with a tech from Dell.  When it finished formatting and installing the op I found I could not get it to configure an internet connection.  In Device Manager, the Ethernet hardware have question marks by them and are highlighted in yellow even though I had put in the ResourceCD and downloaded that and everything on all the other disks right after installation.  If I could get it on the internet I could download the drivers and other things it needs for the other minor problems I am having with the too large icons, etc.  The only difference between the Dell Dimension 2400 and my Dell D 2200 is that the 2400 came internet ready from the factory and I added the ethernet card to my Dell Dimension 2200, Linksys.  Otherwise, they both use the same Broadband, Roadrunner, that is always on with cable modems.  What can I do now?
Need help with reinstalling Windows XP on a Dell Dimension 2400 desktop