While updating & activating windows 7 this error code "0x80072f8f" message can appear due to some reasons:
1. You shouldVerify that your Internet connection is working.
2. Check system time and date to be correct. some times, date changes to an old date due to a battery problem.
3. Verify that your proxy settings are correct, to reach the correct Microsoft server.
If neither of these option succeed then you can used another method to activate your windows
1 - Click Start , and choose Computer.
2 - Click System properties on the above toolbar, you will see below "Click here to activate Windows now" in the Windows activation section, sometimes it requires an administration confirmation, if it does please confirm.
3 - Choose "Show me other ways to Activate".
4 - Click Use the automated phone system, and then follow the instructions that are provided.
After trying these methods, my windows was activated. wish it helps you too.