On a working PC with Internet access, go to http://windows.microsoft.com/en-US/windows/what-is-windows-defender-offline/ and download a 32 or 64-bit version of Windows Defender Offline Tool. It is a small installer file. You still need Internet connection in the next step.
2. Run the Program
After downloading run the program and you will be greeted with the similar screen, as shown in the screenshot below. Then click next and accept the license agreement.
You will be asked where to install the tool – on a CD/DVD, USB flash drive, or to create an ISO file.
3. Make a Windows Defender Offline Tool Bootable Media
Insert a blank write-able CD or DVD in your disk. If you are installing the tool on a USB flash drive, please make sure you backup all data on your flash drive. The USB flash drive will be formatted and all data in it will be lost.
4. Wait until the Bootable Media Created Successfully
Please keep your PC connected to the Internet. The latest antivirus definition files will be downloaded from the Internet and added to your device. The downloaded file size is about 250MB.
5. Boot Infected PC with Windows Defender Offline Tool
Set the infected PC to boot from CD or USB in BIOS. Insert the Bootable Media to the infected PC. Now you can boot up using the media and you’ll get the Windows Defender Offline Tool screen.
The tool will scan your Windows system. It will fix errors and clean malicious software from the infected PC.
Hopefully, the next time you have a problem with your computer and you know it’s spyware or malware, you can use the offline tool to disinfect your system. However if you want to use this tool again then you have to go through the same process again as the files are updated regularly.