What to do to secure PC beyond anti-virus software and a firewall?

I’m a little paranoid after a rootkit turned up on my PC a couple of weeks ago, and I have no idea how it got there or how long it was there before my anti-virus software identified it. Beyond up-to-date anti-virus software and a firewall, what can I do to help ensure security and privacy on my PC?

Read more: What to do to secure PC beyond anti-virus software and a firewall?

Story added 13. December 2013, content source with full text you can find at link above.