from http://www.mydigitallife.info/2009/09/24/how-to-backup-and-restore-windows-7-and-server-2008-r2-activation-status-activate-offline-on-reinstall/ we read:
Try the following trick to backup activation files for Windows 7 or Windows Server 2008 R2, which has been previously been activated, prior to reinstallation. Note that the workaround should only be tried by users who want to clean install Windows, as if user (i.e. want to upgrade to bigger hard disk) makes a clone image of physical hard disk with Norton Ghost or Acronis True Image, all application programs, data, resources together with activation status will be automatically preserved.
1. Copy and save or backup the following activation-related files to external storage medium such as USB flash drive or portable hard disk drive:
Note: For 64-bit (x64) OS, %SystemDrive%\Windows\SysWOW64\spp\tokens\pkeyconfig\pkeyconfig.xrm-ms have to be backed up too.
2. Retrieve and record the product key used to install and activate the current Windows 7 or Windows Server 2008 R2.
Tip: If you can’t remember the product key used, there is plenty of product key viewers available to help.
3. Reinstall Windows 7 or Windows Server 2008 R2. When installation wizard prompts for a product key for activation, leave it blank (do not enter anything).
4. In the newly installed Windows operating system, stop the Software Protection Service in Services.msc or with the following command (run in elevated command prompt):
net stop sppsvc
5. Navigate to the following folder:
Note: In 64-bit (x64) operating system, also perform the action in %SystemDrive%\Windows\SysWOW64\spp\tokens\pkeyconfig\ folder.
6. Take ownership and give user full control permissions (alternatively add grant full control right click menu item) to pkeyconfig.xrm-ms file.
7. Delete the original default pkeyconfig.xrm-ms file, and replace with the backup copy.
8. Navigate to the following folder:
9. Take ownership and give user full control permissions (alternatively add grant full control right click menu item) to tokens.dat file.
10. Delete the original default tokens.dat file, and replace with the backup copy.
11. Restart the Software Protection Service in Services.msc or with the following command (run in elevated command prompt):
net start sppsvc
12. Register the product key for Windows 7 or Windows Server 2008 R2 with the following command (run in elevated command prompt):
slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
Replace xxxxx-xxxxx-xxxxx-xxxxx-xxxxx with the actual product key.
13. Windows will activated instantly, off-line. To check activation status, uses of of the following commands:
Note that the ability to backup and restore old activation status in order to pre-activate or ready-activate a machine after re-installation must be on the same hardware without any hardware change, else user will need to activate online again. Besides, the product key used must be the same serial code too. The hack also does not apply to OEM branded computer which has Windows pre-activated and KMS host activated Windows system.