Passwords and firewalls

Put a strong password on your computer. Use a phrase or initials from the phrase, i.e.
"The lady doth protest too much, methinks" can be used or "Tldptm,m" (if you are allowed non-alpha characters.)

On a hostile network, which is just about any public network, you need to make sure you are not running services and turn on the firewall just in case. Apple's firewall isn't very clear as to what the options mean, so do a little searching if you are unsure. The Windows firewall is pretty clear as to what services are exposed, so it might be a little easier. Basic rule in either case is, if you don't have any open ports, then nobody can connect to your computer.