Hi there are 3 programs that would do it for you, actually 4.
the official way is:
using the Windows .ISO to USB tool from microsoft
http://www.microsoftstore.com/store/msusa/html/pbPage.Help_Win7_usbdvd_dwnTool
Note: this is 1 .ISO per usb.
Now if you want to run multiple .ISO to boot seperatly on the same USB with out partioning it, i'd recommend these programs to use.
SARDU -
http://www.sarducd.it/
YUMI -
http://www.pendrivelinux.com/yumi-multiboot-usb-creator/
XBOOT-
http://www.pendrivelinux.com/yumi-multiboot-usb-creator/ it is in the same webpage.
i have used them all, and i found SARDU to be the best, i have a USB setup with all my diagnostic tools, windows xp, windows 7 that all boot from the one usb.
also if you want you can search up on google and read more about them, just remember to type in Multiboot windows on usb.
it is also worth mentioning that SARDU contains a virtual boot up machine to test your created multiboot usb as well as .ISO files and also an .ISO creater can be found on SARDU's website if you need to create it.
Also, i would format USB in NTFS as the windows setup/installers run better off that file type.
basically it's like an all in 1 tool USB.
hope this helps
