Instead of risking my sanity by trying to install Virtual PC 2004 on my x64 box, I decided to go with Virtual Server 2005 R2 x64. Thankfully, this new release of Virtual Server allows installation on an XP host, and the setup experience was pleasantly uneventful.
Of course I ran into a snag - my default browser is Firefox, and the administration Web site didn't fully function with it. So back to Internet Explorer, and configure the first (existing) virtual machine:
I learned the following things:
Other than that I have to say that Virtual Server 2005 R2 is a much better experience than Virtual PC 2004.