Two cents to start securing your Linux Ubuntu Intrepid box: A firewall and an antivirus.
- Install a personal firewall e.g. firestarter
- $sudo apt-get install firestarter will install it (more here)
- More on how to use it here
- I recommend to run it at bootup time for all users (including the non-admin ones). Go to system>preferences>sessions and add /usr/sbin/firestarter
- For the non-admin ones, you need to edit the sudoers file as indicated here
- Use the System > Preferences > Session to include the command sudo firestarter
- Install an antivirus such as clamav
- $sudo apt-get install clamav-daemon
- More on how to do it (checking that it runs, scheduling an update, etc) here
Those are the first steps to keep the machine secure!