I found a way to add Omniweb to the list of browsers in the Internet Prefpane (see this hint) but this didn't solve my problem. I still had to go back to this Prefpane regularly to set back Omniweb as my default browser. So I had to find a way to keep Internet Explorer on my hard disk but without the system noticing it.
I created a disk image of 15MB and I copied Internet Explorer onto it. Whenever I need Explorer, I mount this disk image and launch the browser. When I'm done, I unmount it. Now even if the system has put back Internet Explorer as the default browser, if I click on a link it will try to launch Microsoft's brower. However, since it won't find it, the system will launch the next browser listed in my Internet Prefpane, namely Omniweb.
[Editor's note: A creative solution to this "all powerful Explorer" problem!]

