OpaTool

Source
This tiny tool works on both Windows 7 and Windows Vista, it allows you to:

• Check the activation status.
• Install a certificate.
• Install a key.

Download current version Current version: 0.7
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Alternatives
There are alternatives to OpaTool, here are some of them.
The manual way:
Do it yourself! It's the safest and the most reliable way to get the job done.
› Take a look at section 6-E and 6-F of the guide for step-by-step instructions.
Use the Windows 7 Loader from Daz:
It's true that this tool is mainly about loader, but it also offers to install a certificate and a key without loader.
› Just tick the checkbox "Advanced options" to access the second panel. It allows you to only install a certificate and a key without installing a loader.
Use My Digital Act¡vator from nononsence:
This tool is specifically made to only install a certificate and a key.