Ubuntu supports WABI installs, which adds an item to the Windows boot.ini for dual booting. Do Mint and Xubuntu support that too?
Ubuntu/Xubuntu aren't different base distros. As far as I'm aware, Xubuntu through Wubi amounts to installing Ubuntu through Wubi, then installing the xubuntu-desktop package.
Mint - there is something called Mint4Win on the install discs apparently, looks like Simon would require using a CD mounting tool* and running it, I have no idea whether it works or how well it is maintained.
*You could use Microsoft Virtual CD control panel for that.