You may find yourself facing activation problems after reinstalling Windows 7. There seems to be a bug in Windows 7 that fails to recognize the key you used (sticker on your computer). This can be due to several problems of which one is the ability to activate properly. This can, as an example, be because of a poor internet connection or security settings on your computer that have become corrupted.
Step one is to try and validate it online instead of using Windows Activation. All you have to do is to click the following link and see what happens - http://www.microsoft.com/genuine/validate
If it fails you can run the Microsoft Activation Diagnostics (you will usually be taken to this page if validation fails) - http://www.microsoft.com/genuine/diag you will usually get advice on what may be wrong as well as advice on what to do. If you get all green try to activate again.
If you still have problems activating your Windows go to the following website and follow the instructions - http://windows.microsoft.com/en-US/windows/help/genuine/submit-request