Windows Defender Offline “helps protect your PC by scanning it to remove rootkits and other advanced malware that can't always be detected by antimalware programs. If this type of malware is detected on your PC, you'll be prompted in Microsoft Security Essentials or Windows Defender to download and run Windows Defender Offline”
And we have a good FAQ to help answer your most common questions
- What if I get an error message when trying to create a bootable CD, DVD, or USB flash drive?
- How do I start my PC from a CD, DVD, or USB flash drive?
- How do I use the Windows Defender Offline ISO file?
- I've used Windows Defender Offline before, can I re-use the CD or DVD that I created?
Also check our Windows Defender Offline system requirements before you start
Learn more about Security, privacy, and user accounts in Windows