How do I activate Windows 7 for a different computer?

I bought Windows 7 Professional Retail which I installed on my laptop. Now I've decided to move it to be on my desktop PC, so I uninstalled Windows from my laptop, restoring the original OS that came with the laptop (Vista Business), and installed Windows 7 on my desktop PC. The problem is that I cannot activate my copy of Windows 7 on the desktop PC. Every time I try to activate it I get the error message:

The product key you typed cannot be used to activate Windows on this computer

and the code of the error is:

Code: 0xc004c008

Description:
The activation server determined that the specified product key could not be used.

How can I solve this problem?

Answer

I solved this issue by calling the automated tool free phone number service for the country where I live and just followed the instructions.
Thank you all who tried to help me.

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1. Press the Windows Logo Key+R. This will bring up the Run dialog box.


2. In the Open box, type "slui 4" and press OK. This will start the
   manual activation wizard.


3. Select your Country in the drop down menu and click Next.


4. The wizard will display a Toll Free phone number. Leave the wizard open on the computer and call that phone number.