avoiding viruses, spyware, malware and adware

It is best to run different types of protection, always running anti-virus program and anti-spy/mal/ad-ware programs as needed.

Use a Good Antivirus Program

Install update and run often, a good anti-virus program such as Avast or Avira. This is based on independent testing from AV Comparatives. As for paid anti-virus I prefer Trend Micro's Worry-Free Business Security Services.

Anti-spyware-malware-adware

Install, update and run on a regular basis antimalware software. My favorites are Malwarebytes and Superantispyware both have free versions. Best to run both at least once a month.

Every month or so run an online antivirus scan like Trend Micro's Housecall.

Once a year download and reinstall all of the anti-virus products you use.