When you use Dell OEM media on a Dell computer, it will check the BIOS to be sure it is installing on a Dell, then it will activate automatically, using a generic OEM key.  As long as you have the COA/license/Product Key sticker, you have the right to run Windows on that computer.  This is pretty standard for OEM systems. System Properties always shows a "code", not the actual key.  You can use Magic Jelly Bean to retrieve the key used to activate Windows, but the key it will return will not match your COA and cannot be used for manual activation.
My Dimension 9200 was in need of a new HDD. Ordered from Dell, installed HD then re-installed WinXP Pro sp2. Forgot to disconnect the media reader so I had to reinstall again to get HDD letter assignment as C. Added drivers in proper order and just finished downloading and installing sp3 and all the updates necessary. Now all that is left is to transfer over any files from the old HD. I was never prompted for the WinXP key code. I DO have the OEM disk from Dell along with the key code from the original Dell factory installation. I was wondering if anyone else had the same "problem". I checked under admin tools and don't see anything that asks for "activation". The only thing I did activate and was prompted for was the key code when I installed MS Office 2003. When I check in System Properties, General, it shows that it is registered to me and some numbers with Dell OEM then more numbers, not the OS key code though.  Can anyone shed light on this? Thanks
reinstalled WinXp Pro on new HDD, not prompted for key code