Malwarebytes Anti-Malware (also called MBAM) is a free and very effective antimalware program. It does have a paid version that adds real-time protection and scheduled scanning, but I will focus on the free version for home users.
If you do not make difference between viruses and malware, please read the What is malware? article.
Open Malwarebytes Anti-Malware Free download page and click the Get my free download button.
The button either starts download right away or takes you to CNet (or some other) download page. In the latter case, click Download Now link.
After dowloading and launching, Malwarebytes Anti-Malware setup starts with asking the language you want to use. English is fine, so click OK.
If something seems to be blocking the download or installation, run RKill to terminate malware processes and services that might interfere with downloading or installing Malwarebytes Anti-Malware. Then retry the installation again, without rebooting your PC.
Next, a typical welcome screen is displayed, click Next.
In License Agreement page, click to tick the I accept the agreement check box. Then click Next.
The change log page called Information is for die-hard fans and geeks. Click Next.
As default destination folder is fine, click Next.
Start menu folder is good, too. Guess which button to click! Yep, Next is the correct answer.
You do not really need Malwarebytes Anti-Malware icons on Desktop or Quick Launch Toolbar. Clear both check boxes in Select Additional Tasks page and click Next.
Finally, ready to install! Click Install.
After file copying is complete, Malwarebytes Anti-Malware setup program asks if you want to update definitions file and launch the main program. Sometimes the Enable free trial of Malwarebytes Anti-Malware PRO option is displayed - clear it.
Leave Update Malwarebytes Anti-Malware and Launch Malwarebytes Anti-Malware check boxes ticked and click Finish.
Some version upgrades might require a restart instead. Close all open programs first.