Depending on which operating system your computer is running, you can use either Windows Defender or Microsoft Security Essentials to get rid of malicious software and viruses.
If your computer is running Windows 8, you can use the built-in Windows Defender to help you get rid of viruses, spyware, or other malware. If your computer is running Windows 7, Windows Vista, or Windows XP, Windows Defender only removes spyware. To get rid of viruses and other malware, including spyware, on Windows 7, Windows Vista, and Windows XP, you can download Microsoft Security Essentials for free.
In Windows 8, Windows Defender replaces Microsoft Security Essentials
You can’t use Microsoft Security Essentials with Windows 8 or Windows RT, but you don’t need to because Windows Defender already provides built-in protection.
What is Windows Defender Offline?
Some malicious software will not allow you to access Windows Defender or other antivirus software. To help detect and remove the malware, you can start your computer by using a Windows Defender Offline CD, DVD, or USB flash drive.