Booting Windows 7 med USB.
This page describes very well how to create a WIndows 7 bootable USB key.
I tried this and had some problems, so I found another way to do it.
I found that NTFS is important. It seems that Win7 will not create the bootable sector on FAT32.
Below the target USB is the USB key you want to boot the WIndows 7 installation from, lets say it's drive
H:. The source USB/DVD is where you have the Windows 7 installation that is not a bootable USB, lets say its drive
I:.
To enable NTFS you must set the device optimized for speed, rather than optimized as removable.
To enable NTFS on a target USB, you should right click the target USB device in the Windows explorer. Select properties. Select Hardware tab, select the correct device from the list of disk devices, and click the properties botton. On the policies tab in the new dialogue, you select optimize for speed. Click OK or Accept until you are back to Windows Explorer.
Now you can right click and select format for the target USB device. NTFS should now exist as a possible filesystem.
Assuming you have the Windows 7 installation copied to your harddrive or you have it on a source device, you use the Windows Explorer to go to the /boot directory on the DVD under the Windows 7 installation DVD.
C:> I:
I:> cd /boot
I:/boot> bootsect /nt60 H:
Target volumes will be updated with BOOTMGR compatible bootcode.
H: (\\?\Volume{aa66dee0-fc4e-11dd-8df0-0014a48e0b5e})
Successfully updated NTFS filesystem bootcode.
Bootcode was successfully updated on all targeted volumes.
I:\boot>
Now you can copy the Windows 7 installation to the H: drive and boot from it, assuming you enable USB boot on the target computer.