Windows Defender Offline is a standalone version of Windows Defender useful for Windows XP and 2003 users as well as offline use. Microsoft Security Essentials has replaced this after Windows XP and 2003.

Essentially it is Windows Defender ready to setup on a thumb drive, CD or DVD or just as an ISO. Now that you have Defender on a portable device you can boot from it and do a scan before anything has loaded.

You should download and create your media using a second, known clean, machine to avoid moving malware from an infected machine to your media.

Once you execute it, it will walk you through the steps needed to create whatever media you choose. From there, insert your CD, DVD or thumb drive, and reboot.

Remember to check frequently for new definition updates so you always have the latest protection. Techs will want to download both the 32-bit and 64-bit versions as you should have the same architecture as the machine you're running it on.