Here's another great tip from the Manageability blog, If you planning on doing a little work with bootable flash drive media you'll want to read this first: Issue: After creating a bootable Task Sequence Media on a USB Flash Drive, the USB Flash Drive does not work and will not boot up on a PC. You may also see the following error messages when trying to boot from the USB Flash Drive: Error message: ---------------Windows Boot Manager---------------- Windows failed to start. A recent hardware...(read more)

Read the complete post at http://scug.be/blogs/sccm/archive/2008/09/05/configmgr-2007-unable-to-boot-from-usb-flash-drive-task-sequence-media.aspx