This installs Windows 7 x86 to a USB key (8Gb or better) or to an External Hard Drive. This means you can carry about a FULL BLOWN Windows 7 on a USB key | Drive.
You don't need to be a Genius to get this to work :)
I've used this myself and had 100% success with about 20 builds with the previous versions and this version (testing).

How to Use :
Check you have a USB HD or key 8Gb or above with 15Mb minimum transfer speed
Run Install.bat

Installation
Install Windows 7 Ultimate to your USB stick or drive, take it with you - and run from any computer (even Macs). Bring your entire operating system, desktop, programs, games, files, personal settings, and much more with you - everywhere! You can now carry your personal operating system in a pocket!

This is a BETA version of Windows 7 USB Edition v4 - it is fully functional but not tweaked in any way. It may be slow and unreliable. A lot of features from previous versions including the cloning utility is missing from this beta ( and will be added later).

New features include:
- You can now install from a DVD (even without Windows!) as well!
- You can install to an internal hard drive as well!

Requirements:
-at least 8GB high speed (15+ MB/s) USB drive or stick
-.Net Framework 2 - http://ub3r.1337.cx/net
[Tested on Windows 7/XP/Vista/2008 R2/2003] - Can be installed from Mac, too - read the included guide!

Installation guide:

-||-In order to install to an external USB/eSata drive or stick:
1.a) Right click installer.bat and choose 'run as administrator'. Follow the on-screen instructions to install Windows 7 USB Edition.
2.a) Insert the USB drive into (any) computer [without any hubs!], (re)start it and press F8, F9, F11, F12, option or another key depending on your hardware. A menu should appear, choose your USB drive from it. You can also change the
boot device order in the bios[press ctrl + s, F2, F10 etc.]. Read the motherboard manual or google it. (This method works on computers that can boot from USB.)
-==OR==-
2.b) Burn the bootloader to an empty CD [run "burn bootloader.bat"], insert the CD and the USB drive to (any) computer, (re)start it and choose "USB" from the menu. (This method will work on almost any computer.)
-==FOR MAC==-
2.c) Install the included rEFIt bootloader on your Mac, insert the drive and reboot.
*******************
-||-In order to install to an internal drive (and use Windows 7 USB Edition as your primary OS!):
1.b) Run 'burn the installer' and burn the installer (or burn '7_USB_Edition_x86_v4-Bootable_Installer.iso' to a DVD/Blu-ray with your favorite iso burning software)
2.d) Start your computer from the burned DVD/Blu-ray and install Windows 7 USB Edition!
3) Wait for the installation to finish - this will take long time if your USB drive isn't very fast.

Instructions for MAC OS

1) Launch Disk Utility (Applications > Utilities).
2) Insert your blank DVD or BD.
2) Drag and drop 7_USB_Edition_..._Bootable_installer.iso file to the left pane in Disk Utility. Now both the blank disk and the .iso should be listed.
3) Select the 7_USB_Edition_..._Bootable_installer.iso, and click on the Burn button. A dialog box confirming the destination (your blank CD/DVD) will appear.
4) Install the included rEFIt
5) Reboot your Mac!
6) Insert the burned DVD/BD, reboot your Mac and immediately press c to start from the DVD/BD
7) Install Windows 7 USB Drive Edition and enjoy it!
8) If your drive is not recognized etc. install it from Windows instead. It is really easier than installing lots of drivers manually - and then start it on your Mac!