There are at least a couple ways you can go about it. To my knowledge there is no sure fire way to boot to a usb drive that works with all equipment.
You didn't say if you had a cd drive. If you do I would first try downloading and burning the iso of "Supergrub." Try using this cd to boot the usb drive. I have had very good luck booting systems in strange places with "Supergrub."
You can also do whats called chainloading from grub. Especially if "Supergrub" works, you can expect chainloading to work. You might want to look at the below thread for more details.viewtopic.php?f=46&t=10043&start=30