open any folder, click Tools, Folder Options, File Types tab, scroll down to REG. Click Advanced. Select Merge in the window that comes up. Click edit.
Action:
Mer&ge
Application used...
regedit.exe "%1"
Use DDE:
checked
DDE Message:
Application:
regedit
DDE Application...
Topic:
System
click ok, the click Set Default, ok, close.