A) Preinstalled by default
B) Available in all Windows 7 SKUs
C) Supporting not just Windows XP, but Win98SE, WinNT4, Win3.1, and DOS 6.22 (for basically complete compatibility with all PC software)
D) Completely transparent (use the compatibility index, age identifiers, etc)
E) Supported hardware graphics acceleration
Edit: And I realize that Virtual PC or VMware satisfies most of these, but then you still have to go through installing all these things, they're still not integrated that well (even in Unity mode).