Using GParted, or another tool, format the USB to NTFS and make the partition bootable.
As root, mount your Windows ISO.
mount /home/osaka/en_windows_7.iso /mnt -o loop
Then copy the contents to the USB.
cp -rv /mnt/* /media/USB sync # Flush file system buffers
Install the Windows boot loader to the USB.
ms-sys -7 /dev/sdX
You may need to download and compile ms-sys.
tar xvf ms-sys-2.X.tar.gz cd ms-sys make make install # As root
Done, reboot with your USB and boot into Windows*…
Links ms-sys homepage
*Still not sure why anyone would want to install Windows :P