Security Essentials for Windows Vista and 7, Windows Defender included in Windows 8). Notes:
- How to activate Windows Defender in Windows 8
- How to Add “Scan with Windows Defender” to the Context Menu in Windows 8
- Windows XP (and earlier) is not secure, should not be used.
SupplementalEnhanced Mitigation Experience Toolkit (EMET) is free Microsoft software that significantly enhances Windows security. Recommended for advanced users.
Since no anti-malware tool is 100% reliable, it's a good idea to supplement Microsoft security by periodically downloading and running a scan with one or preferably more (as no one tool is perfect) of the following: