Protection of pc or laptop from virus or malware is a most common issue these days, but it so easy to handle if you care some necessary steps.
1- Choose stable and some latest version of operating system in case of micrsoft platform try to choose Windows 7 Professioanl or ultimate which is more reliable,stable and protected to destabilize as compare to XP or Vista.
2- Choose Antivirus which is most stable and from long time in market yes it is little bit seem to odd but accroding to my experience Symantec Norton is more updated and provide extreme security from old and latest viruses and malware as compare to new antivirus companies.like eset nod, avast etc.
3- When ever you install new operating system immediately install Antivirus and update it.If you install OS and plan to install antivirus later it may be lead you to collateral damge of your OS in furture,because some time you use usb or other external media ,which are cause to spread virus and malware scripts, so it is possible that after it penetrate in your OS ,even you install antivirus with latest update after attack it will be useless to recover it.
4-Be careful when you use external media specially Usb/Flash drives,before opening folders scan it with your antivirus by just right click and scan it.
5-If you didnt find any good antivirus aur cannot purchase, go for free utilities Like AVG or Microsoft essentials,which will be better option in case absence of symantec or other registerd good antivirus.