It looks as though you've accidentally associated html files with Notepad somehow.
Look in your VC directory for the file commandhelp.html (technically, you can use any html file, but this one is probably handy).
1. Right-click the file, select the menu option "Open With"
2. Go down to "Choose default program ..."
3. Select your preferred web browser. Make sure the box "Always use the selected program to open this kind of file" is checked.
(This is probably what got overwritten somehow previously -- if you opened an html page with Notepad, and didn't notice that that box was checked.)
