You CAN BOOT from a USB device if the computer BIOS supports it but note :
You can't boot a Windows OS yet .
For other OS'es you CAN boot from a USB stick or drive if the machine BIOS supports it.
Insert your USB drive / stick into the computer and go to the BIOS menu via setup when you power on the machine.
You can use a Bartpe system or equivalent but for a genuine full windows it isn't possible yet to boot into a windows system yet (at least not outside MS's own labs - and even INSIDE those I'm not sure.
The only "half way" measure is to have a small bootable LINUX system - these DO boot from USB sticks etc etc which will then automatically start up a Windows Virtual machine.
That's about the best you can do -- and its not such a bad idea either.
32 GB USB sticks are available so no prob in creating a Windows 7 x-64 virtual machine on the same stick as you boot the Linux system from.
I take a small 320GB USB "Passport" drive to work for messing around with w2K3 or W2k8 servers at times without having to change anything on an existing OS. The disk boots nicely (SUSE Linux 11.1) and starts vmware server to launch the windows virtual machines.