Always use notepad, or some other tool all the time, and just want a default 'open with' function? Easy.
* open the registry edit (start, run, regedit)
* navigate to [hkey_classes_root\*\shell\
* create a folder called 'open with XXXXX'
* create a folder called 'command'
* click on defalt
* add command, for example 'notepad.exe %1'