I have an ASUS eeepc netbook that came with Windows. I run Ubuntu on it as
well from a 16 gb SD card. I use the boot manager to handle the chores of
switching which device to boot from. I press esc when it first boots an
choose the sd card instead of the hard drive. As far as Windows knows it is
still on the HD and the only OS installed. The trick i doing it this way is
to write grub to the SD card and not sda. In Ubuntu you do this on page 7
(review page) of the installation by clicking the advanced button.