Help: windows 7 activation product...

HI, there:
I bought a Dell Laptop with windows 7 home premium a month ago and I reinstalled the system with the windows 7 DVD came with the Laptop. However I don't know I need to activate the windows 7 at that time.
Now after 30 days, I have a problem that, the windows message says that, my activation period has been expired. Even when I tried to input the product key on the Dell laptop, it is said that the product key is invalid.
Would you please tell me what to do except reinstalling the windows 7 if reinstallation can solve this problem.
Thank you.