Once your USB drive is ready, download Portable VirtualBox. Open the file and extract the software to your USB drive. (You can also extract the files to another directory, and then paste the newly created folder into your USB drive.) Now, look in that directory and open "Portable-VirtualBox.exe."
A window will pop up, with a huge "Download installation files of VirtualBox" button at the top. Click this button. While you're waiting, click the "Extract files" box for 32-bit or 64-bit operating systems, depending on the computers you're planning to use with VirtualBox. You can check both boxes, but this will take up more space. Check the"Start Portable-VirtualBox after the extract and/or compress" box.
Here's one potentially confusing part of this process: Once the files are finished downloading, click the OK button in the bottom left corner. Do not click the Exit button on the bottom right corner, and do not click the new VirtualBox.exe file that has appeared in your installation directory. (If you see an installation wizard, you're on the wrong track. Re-open Portable-VirtualBox.exe, and use the Search button to select that VirtualBox.exe file, then click OK.)
Be patient after clicking the OK button. It can take several minutes for Portable VirtualBox to extract and install the necessary files, and there's no progress bar to let you know how things are going. You may even hear a few error-like Windows sounds during the installation. Ignore those! If you try to quit prematurely, things can crash and you may have to start all over again. Just sit tight until you see the VirtualBox splash screen pop up.
Once the software is installed, it should open on its own, though it may prompt you to exit and re-open the app to ensure everything's working properly. You can do this by opening the notification area of the Windows taskbar, right-clicking the Portable VirtualBox button, and clicking Quit. You can also tweak settings such as network support from this same right-click menu. (Network and USB support are disabled by default.)
To re-open Portable VirtualBox, click on the same Portable-VirtualBox.exe file on your flash drive--the one that previously led to the setup screen. This time, it'll take you straight to the VirtualBox software.
From here, it's VirtualBox as usual. We won't get into the nitty-gritty of how to set up a virtual machine, but you can check out our detailed instructions on installing Windows XP, Windows 8, Android or SteamOS through VirtualBox. Any of those tutorials should give you a good sense of how Oracle's virtualization software works.
Once you've set up a virtual machine, you can pop that USB stick into any Windows PC and open Portable-VirtualBox.exe, letting you run your chosen OS almost anywhere, and free of the file retention issues associated with ephemeral Linux live CDs. Pat yourself on the back for fitting all kinds of alternative operating systems and aging software in the palm of your hand.
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