Hi Eddie,
I once faced that "HTTP 404" pesky error, and doing a complete re-install made the trick, maybe I missed some installation step in the first installation.
Now, long after I had my BP up and running, a fellow worker of mine stumbled on the following KB entry: "KB923665-How to uninstall Business Portal 3.0"
Although *it says it applies* to BP 3.0, she was told by MSFT support staff that also applies to Biz Portal 5.1. Maybe you should tried it.
I'm assuming you're using ENGLISH installation media for both GP and adtt'l products like web services and BP.
HTH, YMMV