Hi ,
Thank you for posting your query on Microsoft Community.
If you have made substantial hardware changes to your PC, such as replacing your hard drive or motherboard, you need to contact support to activate Windows by phone:
http://support.microsoft.com/kb/950929
Please follow the steps mentioned below to activate Windows on your computer:
a) Click Start, and in the Search box type: slui.exe 4
b) Press ENTER on your Keyboard
c) Select your Country.
d) Opt for Phone Activation option, and seek help from the activation team.
You can also refer to the link mentioned below:
http://support.microsoft.com/kb/950929/en-us
For further queries, please feel free to contact us. We will be more than happy to help you.