Part of my job description entails working as a computer engineer or as an American friend informed me, being a troubleshooter. I've been doing this type of work for about 9 years so have dealt with quite a number of varying computer problems. Recently, though, a friend asked me to remove some viruses from their computer and I was 'blown away' to find around 3,300 viruses! On top of this there was about 200 spyware programs on the computer.
Here are some tips for keeping your computer free from malware (i.e. viruses, trojans, spyware, etc):

• Run CleanUp on your system at least once a week.
• Install a decent virus scanning program and ensure you update the virus definition files at least once a week. Also virus scan your computer at least once a week.
• Install a decent firewall program on your computer.
• Ensure you run Windows Update at least once a week.
• Install a decent anti-spyware program and ensure you update it at least once a week. Also run the anti-spyware program at least once a week.

Update: If you're running Linux, ignore all the previous tips ;-)