The Windows XP Mode Base VHD file (Windows XP Mode base.vhd) is stored in C:\Program Files\Windows XP Mode. The Differencing disk is stored, by default, in C:\Users\%username%\AppData\Local\Microsoft\Windows Virtual PC\Virtual Machines.
The Windows XP Mode Base VHD file (Windows XP Mode base.vhd) is stored in C:\Program Files\Windows XP Mode. The Differencing disk is stored, by default, in C:\Users\%username%\AppData\Local\Microsoft\Windows Virtual PC\Virtual Machines.