Help: windows 7 activation product...

[Effelfadvessy]

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.