Computer can not be infected by viruses if user doesn’t open any files or emails.
Some years ago it was true. But today some viruses use more than one method of attack. They find the joint in the armour of different software such as Windows and Internet Explorer. Such viruses find new victims in the global network on their own.

To protect your computer the only thing you need is to install antivirus software.
To detect new viruses your antivirus software has to have their signatures. So you need to update it routinely. But this fact doesn’t guarantee perfect security. To reduce threat you have to install firewall and special software to detect spy applications.

Emails from well known addressee are safe.
It is not truth! The most of email viruses use address books of their victims.

I don’t keep my private information on my computer so it isn’t of interest of spy applications.
The most of spy applications collect not only private information (such as passwords and credit card numbers) but information about users’ actions. All sites you visit and all clicks are registered. This information can be used for different purposes for example context ads. Such applications use system sources and slow down computer performance.