You could save an autorun.bat on the usb stick, but that could conflict with autorun stuff that it already does, and for all I know that wouldn't work with usb devices...
... If it did, though, you'd just need to have it do something like...

@echo off
start <path to nvda>/nvda.exe
exit

And save it in the top directory of the device.
But again, I have no idea if that'd actually work. So I'd probably just do what dan_c suggested.
windows + m, computer / my computer (whichever the computer in question has), find the drive of the device, then you should be able to find the nvda file.

Or you can be active yourself and look for the portable version on NVDA's website directly. It's not burried anywhere deep within the structure so it's not difficult to find, you know. They also have instructions on how to create that autorun file there somewhere. I've never needed it so I can't be of more help in this regard, sorry, but with the help of that document, you can do it. It might be complicated and you may have to decide between several possible choices, depending on your computer's configuration, but still it can be done and all the possibilities are explained there, as far as I remember.
Lukas