I did this on a Windows 7 machine. It should work on any version of windows that has diskpart. So go ahead and plug in the flash drive you want to make bootable (note that this process will wipe the contents of the drive you select to use)
Open Command Prompt
Now that you are in diskpart we need to identify the flash drive.
DISKPART> list disk
Gives you a list and status of drives available to use. In my case disk 0 is my hard drive and disk 1 is my flash drive. If yours is a different number use the number for the flash drive you want to be bootable.
DISKPART> select disk 1
This tells diskpart we are working with the flash drive.
This wipes the drive we previously selected.
DISKPART> create partition primary
Tells diskpart to create a primary partition on the previously selected drive.
DISKPART> select partition 1
Says to select partition 1 on the selected drive. list partition can be used to show a list of partitions on a disk
Makes partition 1 the active partition.
DISKPART> format fs=fat32
Formats a FAT32 filesystem on the active partition. For bootable flash drives this seems to work the best right now anyway.
Assigns a drive letter to this partition.
Now look and see what drive letter your flash drive is and copy the OS disk you want to boot over to the flash drive and you now have a bootable USB flash drive.