It's an 'item' (right frame in Regedit) in the following registry key: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Discardable\PostSetup\ShellNew (left frame in regedit). That's for XP. It might be slightly different in Vista. Searching for Shellnew might find it.
But it's a strange and un-understandable binary content. So all you can do:
1. export the key to a safe place
2. export the key from your correctly working machine
3. import that file on this machine
4. see if it works
But it could well be this totally screws things up. Then delete the whole key and import it again from the backup copy you made in step 1.
I'm sure you don't like the method described in http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=3683376&SiteID=1 (write a C#-program to do it).
Personally, I would start with a repair install of the OS and see if that repairs the issue.
A workaround might be to put a shortcut to Notepad (starting in your Documents or whereever you want to put your note) in Quick Launch area. Just click that in stead of what you do now.
I've just switched to Vista Ultimate SP1 on both my PCs using same setup disk, and I have a slight problem with one of them.
When you use Right Click and select New (Folder,Shortcut etc) Notepad appears on one machine but not the other. I often use this for note taking, and would be very grateful if somebody could advise me how to add Notepad to the Right Click Menu, so both machines are the same.
Seems odd the setup on one machine is different than the other as I just accepted all the defaults on installation!