I don't think that's possible in most cases.. because most apps will install stuff in the registry, and won't run properly/at all without this information.
Unless, of course, you figure out where the registry entries are stored (best guess is HKEY_LOCAL_MACHINE\SOFTWARE, but it's technically possible for an app to put junk anywhere in the registry) and copy those to your external hard drive as a .reg file.. just add that to the registry of the computer you want to run the app on.
If anyone has better ideas, I'd be glad to hear it..