It simply does not work. Use VMWare or VirtualBox.
You will not get Virtual PC or Virtual Server from microsoft to run x64 Guest VMs.
If it has to be Microsoft, install server 2008 x64 with hyper-v. Make sure you have a CPU that supports Intel-VT or AMD-V depending on brand. there is core 2 and cheap amd cpus out there with virtualization technology builtin for quite affordable prices…
Or do as Mr Don says and run VMware or VirtualBox.
VMware Server(runs on windows) is free tough registration is needed.
Use VirtualBox. It’s free and cross-platform. I’ve used it and it’s an excellent program. VirtualPC is nice, but has quite a lot of limitations.
This will depend what your processor is, but unfortunately most desktops don’t support this. If you have a Core i7, you might be able to get it to work – you’ll need to boot into your BIOS, then find the setting for your processor to enable virtualization. It’s disabled by default – you have to enable it. This should allow you to install a 64-bit OS as a virtual machine. Otherwise you’re limited to 32-bit. It won’t matter which virtualization technology you use, this is the critical piece to get it working with 64-bit.