What is the proper way to undo windbg -I on vista/win7?
Is it as simple as deleting the registry keys in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug?
You don't need to delete the entire registry key. Deleting the Debugger value is enough.
When an exception occurs, the system first looks to see if there is a debugger registered. If so, it just launches the command line specified. Otherwise, it invokes Windows Error Reporting to collect the crash dump. All Windbg -I does is change the registered debugger value in the AeDebug key.