Both Windows Defender Offline and Microsoft Safety Scanner are not a replacement for using a real-time anti-malware (e.g. viruses, spyware, or other malicious software) program that provides full-time protection for your computer. If you need a real-time malware protection check out Microsoft Security Essentials, or any other anti-malware software suite that is available.
Below is a brief explanation of the two offline anti-malware scanners that Microsoft offers. The Microsoft Safety Scanner is designed for scanning your system for malicious programs without having to reboot it. While more difficult strains of malware may require reboot your computer with Windows Defender Offline (via CD or Flash drive) to scan and remove that malicious software.
Microsoft Safety Scanner
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.
Windows Defender Offline
Windows Defender Offline is a bootable CD that allows you scan and remove malicious software from your computer. Since your booting off a CD, it is easier to remove difficult strains of malware that may not be removable any other way.
Note: New threats appear daily, so it's important to always have the most up-to-date definitions installed in Windows Defender Offline. Which means you should always download the latest copy before you using it.