1) Open registry editor (Run... "regedit")
2) Open path My Computer\HKEY_CLASSES_ROOT\*\Shell
3) Right click "Shell" node and select New - Key.
4) Enter the name you want to see in Explorer's right click menu eg "Open with Notepad"
5) Right click on your new node and select New - Key then enter "Command" as it's name.
6) Double click on the name "(Default)" (in the right hand panel).
7) Enter the command to open the selected file in your favourite editor (in the Value Data field) eg:
"notepad.exe" "%1"