Exchange Domain Prep Error: "Microsoft Exchange Server setup cannot continue because a restart from a previous installation or update is pending"

I've come across this error many times, especially when prepping a Child Domain.

The fix is easy:

1. Open Regedit on the machine you're running the prep against

2. Navigate to HKEY_LOCAL_MACHINE\CurrentControlSet\Control\SessionManager\

3. Delete the PendingFileRenameOperations String

4. Rerun your Domain Prep