An easier way to do this: Pick up another copy of windows and put it on that drive with the games, and treat that hard drive as something separate from your everyday PC. When over at a friend's you boot from that drive and everything is kosher. When you're at home you'll have to remember to periodically boot to it to keep everything up to date.
Edit: This works in the context of Steam, in that you'll be able to download your games to both your static PC's HD and your portable HD, with the caveat of having to verify by email whenever you boot from the other, or boot from your friend's house. I wouldn't even try it with Origin, Uplay or any other digital distributor, since they have a habit of permabanning people for logging in from different locations regularly.