Assistive technology doesn’t have to be an “official” Portable App for you to install it on a flash drive, nor does it have to be freeware. (But no, you cannot install Dragon NaturallySpeaking on a flash drive.) It depends primarily on 2 things: the ability to choose the flash drive letter for the install location (creating a folder on the drive if necessary) and, ideally, the option NOT to create any shortcuts on the desktop or start menu. It’s just cleaner that way, though shortcuts won’t hurt anything provided you install from your home computer. However, even after you’ve done this, there is still something you may have to watch out for if you’re trying to use public computers — particularly if those computers are on a queue that assigns you to a random machine, e.g. in a library.
There are 2 main types of licenses when you purchase software: per computer and per user. If you’re going to use your flash drive on a public computer, it’s better if your software has a per user license. This means that the license belongs to you, which in turn means that you can use that software on any computer you operate. The only requirement for using per user software on a flash drive is that when you plug it in, you will need to enter the activation code (and possibly the name you registered it with), so DON’T LOSE THAT. I keep a file on my flash drive containing all of my activation codes, so all I need to do is copy and paste. This step is both necessary and beneficial. It’s necessary because in this setup, nothing gets permanently saved to the registry, which is why you need to activate each time. It’s beneficial for the same reason, especially if you can’t use the same computer each time.
If you have USB software with a license that limits the number of computers you can activate on, trying to use different computers becomes dodgy if not impossible. Shades of per-computer enter here, because the program has to count the installation on each machine. When you activate computer-specific software, it saves that activation to the registry — or at least tries to — so that each time you plug your flash drive into that computer, you can run the software without activating. This is problematic on public machines for a couple of reasons.
First, depending on the security settings of the public computer you’re trying to use, it may not like your attempts to modify the registry by activating the software. The software may thus crash or otherwise not run properly. I suspect this is what happened with Typing Assistant, and is partially why I sought other software. Typing Assistant limits its USB version to 3 computers. Even if you get the software to run properly, that kind of license assumes that you’ll always be able to use the same 3 computers. This does not allow for use in, say, a library where you are assigned to available machines at random. You will not always get the same 3 machines. Therefore, you will most likely be out of luck, which kind of defeats the purpose of portable software.
Per computer license agreements are almost always specified at the outset, so that you can decide if that works for your circumstances. Sometimes, per user licenses are also specified, but sometimes they aren’t. For example, I didn’t know Turbo Type had a good, portable per user license until I copied the program folder to my flash drive and tried to use it a couple of times stick typing on friends’ machines, and was pleasantly surprised. It may be that per user licenses are a given unless otherwise specified, but I would be careful in making that assumption. The best thing to do is try the demo of your program, if one exists, because it will usually explain the license to you in case you want to buy it when your time is up.