Microsoft Security Essentials:
Microsoft Security Essentials is a free product available to download to help defend computers running Windows 7 against viruses, spyware, and other malicious software.
Download at https://support.microsoft.com/en-us/help/14210/security-essentials-download.
Microsoft Security Essentials was replaced by Windows Defender, starting in Windows 8 and later. Windows Defender runs in the background, notifies users to take specific action when needed, and can be used anytime to scan for malware.
Windows Defender is built into Windows 8 and Windows 10.
Tiger Tech also recommends Malwarebytes: https://www.malwarebytes.com/.
To use either of the programs, open programs, select the software located on the machine, and run accordingly.