What additional security measures should I apply for Ubuntu?

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http://askubuntu.com – So on my Windows 7 install, I've got multiple free malware/virus scanners and a firewall up. But I've heard that Ubuntu (or Linux in general) is much more secure by default. So what additional security measures should I take, other than the obvious precautionary measures (aka avoiding shady PPAs)? Should I install a lightweight firewall or something of the sort? Or is the security I get with Ubuntu out-of-box going to be enough for daily web-browsing? I'd rather not have to install too much software, I'd like to keep this PC running as fast and light-weight as possible. Also, if I should hav (HowTos)