I'm no expert when it comes to security, but a few thoughts:
Never install software from outside of the repositories. The Linux idea of all software being held in safe repositories instead of being installed from anywhere is one of its big advantages with regards to security.
Encrypt your home folder. Run Tiger. This isn't an antivirus tool, but it will give you a nice report of some things (never all!) that you might want to look at.
Start a terminal, and type
sudo apt-get install tiger
to get it (you'll need to put in your password)
then type
tiger
Security in Linux is really a mindset, rather than a tool you pay extra for like you do with Windows.
Give me a shout if you get stuck, I'll help if I can.
