Couple things off the top.....
# Run Update Manager and apply security updates always.
# Research a little about having ring on with password.
# AppArmor has recently been added to Ubuntu
http://en.wikipedia.org/wiki/AppArmor (similar in a way to Vista UAC)
# Use Synaptics Package Manager to find/add for SSL or HTTPS communications
# Definately use a firewall minimumly blocking Echo Requests (Firestarter is
simple and does by default)
# Install ClamAV and all packages available at Synaptics Package Manager and set
for automatic updates at least once every 24 hours.
# If desired, there are a couple antivirus linux products with real time
protection as Eset.