Changing the IE7 View Source Editor
It turns out that you can change the editor that IE7 uses to display HTML when you choose View Source.
It uses the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\View Source Editor\Editor Name
I had to go into Internet Explorer and first add key View Source Editor, and then within that add key Editor Name, and then set the (Default) value in there as the path to gvim.
Here's the original MSDN article (which will probably disappear soon, as usual):