When you did the install it should have detected Windows and offered to install along side it. Is that what happened? What I'm sure did happen when you booted from your Windows USB key is that when you ran bootrec, you DIDN'T SPECIFY DRIVE C, so that is why your USB key now boots Windows, you actually "fixed" the boot sequence using the USB drive as the base point. Remember, Windows can be booted from any drive, pretty much on any drive. You can fix this by running bootrec again and specify c: as the target. However, you should also be able to update the grub menu from within Xubuntu as well.
Last edited by guyonearth; 09-07-2012 at 01:59 PM.