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#31
Marking the G: Windows inactive is not all that necessary since you are going to remove the G: shortly, it is just the best course of action.
You can use this excellent little app to retrieve the activation key from G:, just install it to the G: Windows and it will list the key; actually it can be installed and will run from a USB stick, it's a great, free little app.
SIW - System Information for Windows
After you have retrieved the key from G: and are ready to activate the 'new' Windows 7, in the 'old' Windows from an elevated command window type this command slmgr -upk that will remove the key from that install, you will get a conformation.
The key can then be added to the 'new' Windows 7.
How to Activate Windows 7
Then when you're ready to remove G: Windows, use Partition Wizard to do a partition-specific wipe to that partition, have a look at Option Three of this tutorial for an outline of the wipe process.
Partition Wizard : Use the Bootable CD
Do not try to use diskpart as it will not do a single partition, it will do the entire HDD and it is not reversible once run.
Do remember to keep us informed when you have the time.