How to add the Windows defender into Windows Explorer's right click menu to scan a particular drive/folder/file on demand in Windows 8?

Avirk's answer actually doesn't scan a file or folder directly, it just initiates a quick or full scan of your entire computer.

http://www.thewindowsclub.com/add-scan-with-windows-defender-option-context-menu-windows-8 outlines a method to get the scan to work on folders. I think with some modifications it can be made to work on files as well.

UPDATE:
It is fairly easy to get this to work on files, just change the /folder/ part of the registry keys to /*/.

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\*\shell\WindowsDefender]
"MUIVerb"="Scan With Windows Defender"
"Icon"="%ProgramFiles%\\Windows Defender\\EppManifest.dll"
@=""

[HKEY_CLASSES_ROOT\*\shell\WindowsDefender\Command]
@="\"C:\\Program Files\\Windows Defender\\MpCmdRun.exe\" -scan -scantype 3 -SignatureUpdate -file %1"

(Just paste the above in a file with extension.reg and merge it into the registry by double-clicking the file.)

The one thing I can't get working is bringing up the GUI scan versus a console version of the scan output.

I had this guide for adding elements to be context menu by editing the registry, it was written for vista but I believe the same keys exist in windows 7 and 8.

It only references a direct exe though so this may not be what you are looking for.

As to a built in solution? No. Windows 8 defender does not have any context menu options for specific scanning.

Remember though, you can still install any other virus scanner as well, and Windows Defender will take a backseat automatically.

Add Scan with Windows Defender in Window 8 Context Menu

Step 1: Open Windows 8 Run box, type in Regedit32.exe and press enter. You will require administrative privileges to perform the action.

Step 2: In Windows Registry Editor navigate to HKEY_CLASSES_ROOT\folder\shell and create a new sub key in the left hand pane. Name the key WindowsDefender to create a new folder.

Step 3: Select WindowsDefender on the left sidebar and right-click anywhere on the right-hand side to create a new String. Type in %ProgramFiles%\Windows Defender\EppManifest.dll in data field and name it Icon.

Step 4: Having done that, add another String, provide the data as Scan with Windows Defender and name it MUI.

Step 5: Now create a new sub key under WindowsDefender and modify the data of the default DWORD to “C:\Program Files\Windows Defender\MpCmdRun.exe” -scan -scantype 3 -SignatureUpdate -file %1.

Try out the newly added context menu option on a folder.Good Luck!!