Regedit can only view and export from the registry of the active Windows installation. It can't open "offline" registry files from another installation.
There is an "Import" option but that can only import the entire .DAT file and overwrite everything that is there already which doesn't sound like a great idea, at least for this purpose.
You wanna try that again? Using the load hive and unload hive you can open other hives from 'offline' installations. It is located in the file menu. I have done this to find the difference between settings on different machines.*
Now, I don't remember if this works under all flavors of windows. But it will work under NT, 2k, 2k3, XP.
*Load Hive and Unload Hive affect only the HKEY_USERS and HKEY_LOCAL_MACHINE keys and are active only when these predefined keys are selected. When you load a hive into the registry, the hive becomes a subkey of one of these keys.
"I used to think the brain was the most amazing organ in the entire body. Then I realized who was telling me this."
If ignorance were painful, half the posters here would be on morphine drips.