Link to Tutorial
This method works with USB Flash drives - if you are using a USB Hard Disk (or USB Flash drives that appears as a 'Local Disk' in Windows Explorer), you will also need a small USB Flash drive or you will need to type a few commands once the Setup utility has booted. See the tutorial link for full details.
It uses some interesting commands available with the latest (chenall) grub4dos version (as used by RMPrepUSB).
[Edit]Also see Easy2Boot - boot from any Windows Install ISO (XP-->SVR2012) just by adding the ISO files.[/Edit]