Scam artists try to trick people into clicking on links that will download viruses, spyware, and other unwanted software — often by bundling it with popular free downloads. To reduce your risk of downloading malware:
Install and update security software, and use a firewall. Set your security software, internet browser, and operating system (like Windows or Mac OS X) to update automatically.
Don’t change your browser’s security settings. You can minimize "drive-by" or bundled downloads if you keep your browser’s default security settings.
Pay attention to your browser’s security warnings. Many browsers come with built-in security scanners that warn you before you visit an infected webpage or download a malicious file.
Instead of clicking on a link in an email, type the URL of a trusted site directly into your browser. Criminals send emails that appear to be from companies you know and trust. The links may look legitimate, but clicking on them could download malware or send you to a scam site.
Don’t open attachments in emails unless you know who sent it and what it is. Opening the wrong attachment — even if it seems to be from friends or family — can install malware on your computer.
Get well-known software directly from the source. Sites that offer lots of different browsers, PDF readers, and other popular software for free are more likely to include malware.
Read each screen when installing new software. If you don’t recognize a program, or are prompted to install additional “bundled” software, decline the additional program or exit the installation process.
Don’t click on popups or banner ads about your computer’s performance. Scammers insert unwanted software into banner ads that look legitimate, especially ads about your computer’s health. Avoid clicking on these ads if you don’t know the source.
Scan USBs and other external devices before using them. These devices can be infected with malware, especially if you use them in high traffic places, like photo printing stations or public computers.
Talk about safe computing. Tell your friends and family that some online actions can put the computer at risk: clicking on pop-ups, downloading "free" games or programs, opening chain emails, or posting personal information.
Back up your data regularly. Whether it's your taxes, photos, or other documents that are important to you, back up any data that you'd want to keep in case your computer crashes.
source: https://www.consumer.ftc.gov/articles/0011-malware#rid