There are links here and here (among many other places) that shows how to replace notepad editor with the excellent Notepad++ on a vista or xp machine as the system default text editor. If you use vista 64 you'll need to add another directory to the location where you replace the original notepad.exe
So the set is :
As always on Vista you'll need to jump through a few hoops to allow you to do this. The first is to replace the file owner with someone other than the default 'TrueInstaller' and secondly to give the new owner full control over the file.
The above links give more info if you need it.
Don't forget - you mess with windows at your own risk :-)