Something similar to unity mode is available only in the version of Virtual PC shipped with Windows 7. That's how XP mode works, an XP app window will appear alongside all the other Windows 7 windows, but without being themed (it still has the normal Windows XP theme, not the Aero theme).
In the image above you can see that one of the Office windows has the default Windows XP theme. That's similar to unity, it's outside the VM.
Edit: running something similar to Unity is not possible in earlier versions of Virtual PC, it's a feature limited to only the version that ships with Windows 7; this version can't be installed in XP or Vista.