Microsoft Security Essentials is a free antivirus program for Windows 7 and Vista PCs. It cannot be installed with any version of Windows 8. Windows Defender handles the same functionality within Windows 8.  This security update is available for download at www.microsoft.com/en-us/wdsi/defenderupdates.

Microsoft Safety Scanner is a separate free virus scan program that can be downloaded and run once, or as needed. The resulting file is msert.exe, and will expire in 10 days.  It can be downloaded from https://docs.microsoft.com/en-us/windows/security/threat-protection/intelligence/safety-scanner-download