Here is a portable anti-virus for your USB flashdisk : http://portableapps.com/apps/utilities/clamwin_portable
It's free, and it's really powerful.
when you are on your own computer, it tells you that your virus definitions files are not up-to-date and asks you if you want to update them, say yes. And, when you are on your friend's computer, say "no, no update please" and it will do the job.
Of course, the rest of your tutorial is useful, but having a working antivirus is also a good point.
Regards
Yordan