As 'Windows Virtual PC' for Windows 7 only emulates a Intel Pentium III, which only supported 32-bit operations, you won't be able to get an x64 version of Windows 7 to work correctly running as a guest. However you will be able to run Windows XP 64-bit as a guest but it will switch down to 32-bit operation.
The Wikipedia article on the subject adds some more information you might appreciate.
I successfully used VirtualBox (which is free and almost 100% open source) to run a Vista x64 guest on a Windows XP 32 bit host, so I assume that it would work with Win 7 x64 on Win 7 32 bit as well.
One thing you need though: hardware assisted virtualization (ie. VT-x on Intel / Pacifica on AMD).
As 'Windows Virtual PC' for Windows 7 only emulates a Intel Pentium III, which only supported 32-bit operations, you won't be able to get an x64 version of Windows 7 to work correctly running as a guest. However you will be able to run Windows XP 64-bit as a guest but it will switch down to 32-bit operation.
The Wikipedia article on the subject adds some more information you might appreciate.
I successfully used VirtualBox (which is free and almost 100% open source) to run a Vista x64 guest on a Windows XP 32 bit host, so I assume that it would work with Win 7 x64 on Win 7 32 bit as well.
One thing you need though: hardware assisted virtualization (ie. VT-x on Intel / Pacifica on AMD).