You cannot - Virtual PC only supports 32bit Operating Systems as guests - I tried the same thing myself, and that is how I found out - See
Microsoft Virtual PC - Wikipedia, the free encyclopedia
Also see
http://blogs.msdn.com/virtual_pc_guy...d.aspx#1723713 and
http://blogs.msdn.com/virtual_pc_guy...d.aspx#1725682 and
http://blogs.msdn.com/virtual_pc_guy...d.aspx#1733528 and
http://blogs.msdn.com/virtual_pc_guy...d.aspx#1800821
What I did when I found that out was sign up to Beta test VMWare new VMWorkstation. It supports both 32bit OSs and 64bit OSs.