"This copy of windows must be activated"

I got that error today, when I tried to log on. But, my Windows XP has been activated for over 7 years now, and I assure you, I paid for my copy. I can't do anything on my computer, and, I have no idea where my registration key is. So, why is this problem occuring, and how do I fix it?

Thanks for the help, in advance...

You may have to phone up/email Microsoft and give them your name, if your Windows is legit, they'll know it.