Sometimes you may double-click any registry file it opens with Notepad instead of automatically being entered into the registry.You might have accidentally associated the ‘.REG’ files to open with Notepad. You have two ways to overcome this issue. One way is import the registry settings into the registry.
To do this:
Click on ‘Start’ and then ‘Run’. Type in ‘regedit’, and then press ‘OK’
In the registry editor, click on ‘File’, and then ‘Import’. Enter the filename or otherwise locate the ‘.REG’ file you want to import, and press ‘OK’ The contents of that ‘.reg’ file will be entered into the registry.
The other way to reassociate the files is: Right click on the ‘.REG’ file in ‘Windows Explorer’ and then click on ‘Open with’. Click on ‘Registry Editor’ if it's in the list, or use the ‘Browse’ button to locate ‘regedit.exe’ (find it in the ‘C:\Windows’folder)
Make sure that ‘Always use the selected program to open this kind of fi le’ is checked. Click ‘OK’