I often use the method you describe, even though I know HTML pretty well. It's just so much faster. Sometimes I save messages that I know I may want to use again later.
I use the same version of Outlook Express as you mentioned too. I once installed the full Outlook program,but as far as I could tell, it did not have the source button available where I could edit the message HTML.
For that reason only, I uninstalled it and stayed with Express. (perhaps I just missed it - can anyone tell me?)
I have both Outlook and Outlook Express installed. You should be able to run them separately, although only one of them will be your default email program. You may have to restore the shortcut for Outlook Express.