Microsoft's free antivirus comes built into Windows 8 as Windows Defender, but it doesn't have an option to scan individual files by right-clicking on them. Our friends at the How-To Geek show us how to add it (or any other program) to your context menu.
The easiest way to do this is to just run a .reg file, which you can grab from the How-To Geek here. Then, just right-click on a folder in Windows Explorer and choose "Scan with Windows Defender" to scan it for malware.
However, if you want to add it yourself manually — or if you want to add a different program to the context menu — the folks at How-To Geek show you how to do that from the registry too. Hit the link to see how it's done.