There is a 64-bit version of Virtual PC 2007. The application itself is not 64-bit, but the drivers it contains are. You can download it from here: http://www.microsoft.com/downloads/details.aspx?familyid=28C97D22-6EB8-4A09-A7F7-F6C7A1F000B5&displaylang=en. However, before installing it, you may want to update Windows 7 first because Microsoft released application compatability updates today that may address the issue you are having. I have a 64-bit version of Windows 7 as well and after doing this, Virtual PC 2007 was running on my computer without any problems.
If you have hardware-assisted virtualization, then I would download Windows Virtual PC instead. You can download it from here: http://www.microsoft.com/windows/virtual-pc/download.aspx. However, I have never used it before because I do not have hardware-assisted virtualizaiton. Thus, I cannot say for sure if it can handle operating systems other than Windows XP. But I think it can, so I'd still give it a go. I hope this helps!