Some disturbing news yesterday was that viruses and spy ware are now being used by pedophiles to use your machine to harvest child porn, and store it for them.
Obviously, this could get you in a lot of trouble, if the FBI shows up at your house and arrests you for it, and confiscates your computer.
The best way to guard yourself against this is to install an Internet filter on your machine. I have a link to K9 in my sidebar. After installing it, make sure it is set to block AT LEAST "pornography" and "security threats." I would also suggest having it block sites relating to "illegal activity." What this will do is prevent you or any process on your system from accessing these kinds of material without a password, even if you do get infected.
Also, here are my general tips for protecting your PC:
1. Remove paid antivirus software and install free antivirus software. You can't afford to let your subscription to paid antivirus lapse and then fail to be protected for even a short period of time. My suggestion is to remove these and install a free antivirus such as Avira, AVG, or Avast.
2. Install an Ad-Blocker. If you use Firefox, you can try Ad-Block Plus, which allows you to select a server from which it will obtain ad-blocking information. If you are willing to pay or don't mainly use Firefox, consider Ad Muncher, which will block ads from ANY application on your system. Unscrupulous ads are often the gateway to spy ware.
3. All three free antivirus programs above include spy ware protection. For additional spy ware protection, consider installing Spybot Search & Destroy. The disadvantage of this program is that it doesn't manually update itself... you need to make sure to update it maybe once a week, and tell the program to "Immunize" after the update has completed.