XP-PRO-N invalid key

I acquired a Dell desktop that was a company discard with XP Pro-N, and all Microsoft updates through 2007 including IE7, and it works very well. I installed browser S/W and went to windows update to bring it up to date and the first was the WGA tool, which gave me three messages.
1. This version of Windows not available in your country.
2. Not genuine Microsoft. Product key not assigned by Microsoft.
3. We will sell you a key to continue to use it for $149, which is not reasonable because the computer is only worth $150.
Can anyone tell me how a once genuine copy of XP becomes not genuine, or explain this situation to me. The COA is for XP home so the PRO is an upgrade, and the keys don't match. Thanx