Although not a replacement for proper antivirus and security procedures – could be useful in ruling out malware and viral effects so – this is what they have to say about it
Do you think your PC has a virus?
The Microsoft Safety Scanner is a free downloadable security tool that provides on-demand scanning and helps remove viruses, spyware, and other malicious software. It works with your existing antivirus software.
Note: The Microsoft Safety Scanner expires 10 days after being downloaded. To rerun a scan with the latest anti-malware definitions, download and run the Microsoft Safety Scanner again.
The Microsoft Safety Scanner is not a replacement for using an antivirus software program that provides ongoing protection.
For real-time protection that helps to guard your home or small business PCs against viruses, spyware, and other malicious software, download Microsoft Security Essentials.