I wrote that post a while ago, so there a couple of things I've learnt since then. Do not partition the space you want to install a fully virtualized guest os in it, since the guest os will not see the partition anyway, it will just see a physical disk, of undetermined format, so don't bother.
My original question was concerning the problem, of mounting guest os partition into host os, something that is less relevant when you have windows install. If you would want to do that anyway, your partition would look like I drawn above, and in order to mount the main partition you'd have to skip the boot sector, which is easily done using loopback device.
But if you want to learn about xen itself, I suggest you to read the following tutorials:
And something explaining how xen works (in particular paravirtualization engine):