The Update Didn't Work!
You've updated to the latest patch you can find for Windows Defender but you're still running into Windows Defender error code 0x8050800c. Now what can you do about the error? At this stage, its time to start modifying your registry keys.
Caution: Note that registry key modification can break your computer and you may need to reformat your computer, or boot to factory settings if actions are performed incorrectly. If you are uncomfortable with performing the following tasks, reach out to a person more familiar with systems to follow the steps.
Modifying the registry keys is basically manually performing what the patch should have performed. For some reason or another the patch failed at modifying the registry keys correctly and its up to you to do the task.
The first step is to open the Registry Editor. You can open the Registry Editor by going to Start, then Run, and typing in Regedit. You will now have the Registry Editor open for editing your registry keys.
Navigate down to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version\ProfileList. Now click on the plus (+) sign to expand the branch. You'll see a list of keys that start with S-1-5... Click on each one and look at the keys on the right. You will be checking for ProfileImagePath with a type of REG_EXPAND_SZ.
If you see one branch that doesn't have ProfileImagePath, then add it in. This can be done by right clicking in the right hand pane. Highlight over New then click on Expandable String Value. A new item will appear with the name of New Value #1.
The last step is to rename the newly created item. Just Right Click and select Rename. Rename the item to ProfileImagePath.
Perform this on each of the branches that start with S-1-5.
Now close Registry Editor and restart your system. Once your computer is up, start up Microsoft Security Essentials, or Windows Defender and run a scan. You have now fixed Windows Defender Error Code 0x8050800c!