USB MultiBoot 10 is a great utility to make usb flash drive bootable. wimb wrote an exerllent tutorial on this: http://www.boot-land.net/forums/?showtopic=4900. At the time of this writing, there are no newer version (I did found a few USB MultiBoot 12 downloads, which turned out to be trojan).
In the past years, I always kept a USB key with Windows XP, GRML (a linux live CD for sysadmins), and other utils this way.
The problem: Under Windows 7, even with the UAC turned off, it fails after formatting the USB flash drive.
Cause: The script detects the Windows 7 as non-vista and handles it as Windows XP.
Solution: Edit USB_MultiBoot_10.cmd, find the lines:
VER | find "6.0." > nul
IF %errorlevel% EQU 0 SET win_vista=1
and change to:
VER | find " 6." > nul
IF %errorlevel% EQU 0 SET win_vista=1