-updated 05/08/09

Malware, spyware, trojans, viruses, worms, and all the other nasties out there come in many shapes and sizes, and get onto your PC in several different ways.  The purpose of this post is to bullet proof your PC so they do not even get the chance.

1. Download and install Secunia. This program will run constantly in the background and advise you when any program on your computer is missing an update.  Very stable and secure, for Windows only.
2. Change your browser! Stop using Internet Explorer.  Most evil things get onto your computer through your web browser, and IE is not that secure.  Available for all platforms (Windows, Mac, Linux) l suggest using FireFox. 
3. Once you've downloaded Firefox, install the WoT (Web of Trust) addon.  This will place a green or red circle next to every link when you do a Google, MSN, or Yahoo search.  Green is good / safe, red is bad / evil.  If you click on a link that was marked red, you are almost guaranteed to end up with something on your computer that you do not want there!
4. Install a working up to date (and auto updating) anti virus program.   AVG and AVAST! are both great personal and free programs.  The biggest difference is AVG comes with a toolbar/link scanner (which you can hide so it doesn't get in the way) and AVAST! does not.  If your current anti virus program is expired, not working, or you just don't want to pay for it anymore, use one of these. 
5. Bad things like to play and live in your temp files. CC Cleaner is a robust disk cleanup program that runs very quickly and takes care of those files plus more.  Does not run automatically, should be run monthly. 
6. Malwarebytes is one of the top rated programs for keeping malware off your PC.  The free version works great, but does not do live background scanning (you have to run it manually).  Most anti virus programs do NOT find what this program does; this is a must have program.  If you use the free version, depending on your internet usage, we suggest running it once every 2-4 weeks.
7. OpenDNS is like a traffic cop for the internet.  It censors (at your discretion and choice - it is completely customizable) what your computer / network can look at and see on the internet.  Completely free, this is highly suggested for anyone that has children / teenagers and small businesses. Even if you know that little Johnny would "never" intentionally go to a bad site, misspelled words in search engines happen. 
8. Do an occasional online scan.  (this does require using Internet Explorer though); Bitdefender Online scan will find things that your onboard antivirus program may not, especially if there is a trojan that is blocking it from running correctly. 

Good luck, we wish you safe browsing!