Enter GRUB and run:

chainloader (hd1)+1

(Source: http://forums.hak5.org/index.php?s=da398a4c6c795666e0b64146756c4bf6&showtopic=14168&view=findpost&p=140848)

Note that this requires your GRUB bootloader has been built with support for USB. If that's not the case I've heard that Plop Boot Manager can help. (I've not personally tried it.)