Virtual PC 2004 No Help Solving 64-Bit OS Woes

In my continuing quest to install XP-64 on my Athlon laptop, I just tried Virtual PC 2004. I get the message "Attempting to load an X86-64 operating system, however this CPU is not compatible with X86-64 mode." which is misleading at best. There aren't any obvious settings I can change. Anyone know if there's some obscure command-line switch I'm supposed to use?

Meanwhile, I know that my old VMWare Workstation version doesn't support 64-bit OSs, so that's out, too.