Windows' own security tools are among the best available, but they usually require pre-installation and regular updates. If you've got to save a Windows system that's already infected — like, say, your relatives' desktop over the holidays — you can boot Windows Defender from a USB stick while you're offline.
You'll need a working copy of Windows to create the bootable USB stick, so if your primary system is in need of saving, you'll have to find a friend or a spare computer. But the process for creating an official offline Defender system is just a few clicks, and it's probably your best shot at recovering a Windows system that's fallen victim to viruses. The How-To Geek site has the full step-by-step skinny on dedicating a USB stick to Windows Defender's offline version, and the offline Defender is a free download.