Windows XP won't recognize even I enter the right key.?

My Dell CD for XP Service Pack 2 installed XP last month. I ignored the "Activate Windows Now" thing on the bottom right of the screen. So, I go to activate it. Problems: Even after correctly entering the code when I spoke to an operator or entering the Activation Key correctly online, both said they don't recognize it (it is a genuine Windows code). Now when I log on to my computer, it tells me I have to activate Windows. Again, I enter the codes correctly either by phone/online but neither recognize it (in the past when I had to re-install XP, it has worked). Help me, please!