
Active malicious software is malicious software that is currently running on the computer.
The tool focuses on the detection and removal of active malicious software. Specific prevalent malicious software is a small subset of all the malicious software that exists today. The tool removes only specific prevalent malicious software. It is significantly more desirable to block malicious software from running on a computer than to remove it after infection. Antivirus products block malicious software from running on a computer. The tool removes malicious software from an already-infected computer. The Microsoft Malicious Software Removal Tool differs from an antivirus product in three key ways: Therefore, we strongly recommend that you install and use an up-to-date antivirus product. It is strictly a post-infection removal tool.
The Microsoft Malicious Software Removal Tool does not replace an antivirus product.
How the Microsoft Malicious Software Removal Tool differs from an antivirus product I would imagine each has there own specialities and are important in their own rights. I have no idea as they are two different types of scanners.