I started searching for how to do this and found fellow MVP Rick Strahl's post asking if anybody knew how to override Windows File Protection to "permanently" replace Windows Notepad with EditPlus.
Like Rick, I've been using EditPlus since 2001. It has so many features (including the ability to view your work in the browser without leaving the product) that I won't even begin to describe them. Some developers like similar products; this post is about how to replace notepad with ANY Notepad replacement on the Windows Vista OS.
In order to accomplish this little feat, we need to not only take ownership of the resource, we also need to grant ourselves ACL permissions on the file:
1. Open up an Administrator command prompt
2. Take ownership of the old notepad.exe with:
takeown /f c:\windows\system32\notepad.exe
3. Grant yourself full control with (replace "<username>" below with your username):
cacls c:\windows\system32\notepad.exe /G <username>:F
4. Open explorer and replace the notepad.exe file with a renamed version of editplus.exe (or whatever your favorite Notepad replacement is).
I haven't fully tested this yet to see if Windows File Protection will try to overwrite it at some point, but for now, I did reboot, and it looks like it works!