If you’ve been infected by a virus or found your system doesn’t boot any more you may be able to repair your system by booting Windows from a USB drive.
Microsoft has a small OS called Windows PE (WinPE) available from this link: WinPE on Microsoft.com. There are instructions on the linked page to create the ISO which can be imaged to a CD, DVD or USB drive. There are many sites on the web with WinPE images available; be careful, many of these come with undesirable software pre-installed. Take the extra time to download and create the image from Microsoft; it’s not that much of an effort to know your OS is clean. Once you have booted from the USB drive you can easily install SuperAntiSpyware, MalwareBytes or your favorite cleanup and repair tools.
This is not a full OS, it’s specifically designed and licensed for repair and recovery. For IT staff having this on a USB stick ready to go is nearly essential.