Microsoft offers a number of free anti-malware tools. Windows 10 and 8.1 users have Windows Defender. Windows 7 users can chose between the full-featured Microsoft Security Essentials (MSE) or the limited Windows Defender. But all Windows users have access to the Malicious Software Removal Tool (MSRT) even though they may not be aware of it.
MSRT is part of the monthly patch Tuesday bug fixes. Windows Update downloads a new version of MSRT and runs a scan with it as part of its normal processing. The last patch Tuesday was May 9th and Microsoft dutifully issued a new version of MSRT.
I mention this because on May 23rd someone contacted me to ask why MSRT had just been updated. He had seen a new version appear in Windows Update on a 32 bit Windows 7 machine that had been dutifully updated on the 9th.