Install/run Win.7 from USB/External HDD Drive

Hello,
I am running Windows 7 & Firefox 3.5.7.
I want to install and run Windows 7 from a USB stick / HDD drive.
With all the drivers in Win7 this should work as a "plug-in + start-up" ??
Can this be done + HOW !!!!
i'm a Non-Geek, so Instructions need/must be BASIC & step-by-step.
Looking forward to reply,
Many thanks for responses to previous Questions.
Regards,
Jack