|MS doesn't want you to do that.|
Get an internal drive. I'd guess if you read the eula it might say you can't install it like that.
Now one can install it to a iscsi target, dunno how that is different.
There have been some web pages that tell how to do what you want.
I would install linux to the usb drive and run a virtual machine for vista.
1/3 of highway deaths are caused by drunks. The rest are by people who can't drive any better than a drunk.