Virtual PC 2007 SP1 x64 edition installs into the Program Files(x86) directory
When you install Virtual PC 2007 Service Pack 1 on a computer running Windows Vista or Windows 7 x64 edition, the installation appears in a subdirectory of the “Program Files (x86)” directory. Additionally, the VirtualPC.exe process displays in Task Manager as a 32-bit process.
When Virtual PC 2007 is installed on a 64-bit host Operating System it installs to the 32-bit program files directory. This is because unlike Virtual Server, Virtual PC 2007 is a 32-bit application which uses 64-bit drivers when necessary.
In order to run on a 64-bit host Operating System the drivers must be 64-bit themselves, but the only benefits of making the application 64-bit is the ability to access larger amounts of memory and to run more virtual machines at the same time. While both of these things are very important for Virtual Server, they are not as important for Virtual PC.
This works as designed for hosting 32-bit operating systems as guests on x64 hosts.