The Microsoft documentation:
Recover a DAG member Exchange Server
Remove the copy of the Database in a DAG:
This will warn that it cannot communicate with the server. That is expected.
Then, you can remove the server from the DAG:
Reinstall Windows 2008 R2 from DVD (remember, DAG requires Enterprise!)
Reset computer account in Domain (Right click Reset in AD Users and Computers)
Name and IP the server, confirm the date/time is correct (since in a DAG, I also needed to IP my DAG network)
Install Exchange Pre-Requisites
Install Exchange 2010 using:
If you skipped the DAG removal steps above, setup will fail with:
Once setup succeeds, you need to reboot the server (at this point, I would also patch as needed - at the time of this writing 2008 R2 and Exchange 2010 RC have no additional patches)
Since I have a DAG, I am able to re-add it to the database availability group and allow the database to reseed. If you were in a single server environment, this is where your backup would come into play. This might be scenario #3
Add the recovered server back to the DAG
Add a mailbox database copy to the recovered server.
Assuming you have a real DAG and you might have 300GB of data to re-seed by adding a copy, this is where the Windows Server Backup may play part. You may be able to restore the Exchange data to an alternate location, and before you add the database copy, move the restored EDB to the folder path for the database. This would allow you to skip time consuming reseeding, as long as your restored EDB was the most recent backup taken of the database.
Optionally, you can activate the database on the recovered server as well.