What boot menu? Are you referring to the BIOS firmware Boot Options? Are you selecting the windows drive to boot windows, the ubuntu drive to boot ubuntu? When you look at the /boot/grub/grub.cfg file on Xubuntu, do you see a chainloader entry for windows? If so, post it here. If you don't have it, have you run sudo update-grub from Xubuntu since you got both systems running? If not, try it and watch the output to see if a windows entry shows.
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