As far as I know a file is program is portable so long as (a) it doesn't depend on any external files, registry keys, or settings or (B ) carries all the files it needs alongside itself. Clickonce installers aren't by definition portable software, but you can just copy out the .exe file from bin/release after you publish.
This would still depend on having the .net framework installed on the system you're servicing; there's no way to run a VB.net application without .net. If you're going to work on earlier machines it may actually be worth compiling for as early a .net version as you can.
I can't comment on how to code the rest of your app.
Edited by JosiahK, 16 June 2011 - 04:25 PM.