Why do you need to run Windows from the external drive?? Just run Windows from your main drive, and put your roms on the USB one.
What he said...
But Yes it is possible ....
Are We talking of A USB Stick or a External drive with a Hard Drive
enclosed?
External Harddrives are easy pop it open install Windows;
then put it back in the case done deal.
USB sticks can Be done Sort of ...
Google "Bart PE" :
It can boot Windows XP from a pre-made ISO
image on your USB stick.
Of course all this depends on your System specs
and if your computer will Even boot from USB.
Hope that helped clear things up...
*** Teutonic Darkness ***