Unable to start Windows Defender after uninstalling McAfee Trial version

Hello Beth,

As per your description, I understand that the issue you are facing is related with the security of your computer. It seems like the security components have been forcefully altered/disabled by McAfee due to which its not working normally like before. This is the reason why we suggest our users on not installing third party security on Windows 8.1 as they are not giving any reliable protection. Moreover, they hamper Windows Update and Windows Security due to which users faces these kind of issues. I will certainly help you with the issue.

As of now, I recommend you to try the following steps:

Step 1:

Please perform SFC /scan. System File Checker is a utility in Windows that allows users to scan for corruptions in Windows system files and restore corrupted files.

1. Swipe in from the right edge of the screen, and then tap Search. Or, if you are using a mouse, point to the lower-right corner of the screen, and then click Search.

2. Type Command Prompt in the Search box, right-click Command Prompt, and then click Run as administrator.

3. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.

4. At the command prompt, type the following command, and then press ENTER:

 sfc /scannow

5. After the process is finished, you may receive one of the following messages:

Windows Resource Protection did not find any integrity violations.

Windows Resource Protection could not perform the requested operation.

Windows Resource Protection found corrupt files and successfully repaired them.

Windows Resource Protection found corrupt files but was unable to fix some of them.

After performing SFC scan, perform Clean Boot.

Step 2:

Boot the computer in a Clean Boot state and check the status. A clean boot helps to verify if any third party application service or startup item is causing this issue.

Note: After you have finished troubleshooting, follow steps under “How to reset the computer to start as usual after troubleshooting with clean boot” in the article to boot to normal startup.

Step 3:

If issue continues, run a full computer scan with the Microsoft Safety Scanner to make sure that the computer is free from virus. 

Refer to the Link:

http://www.microsoft.com/security/scanner/en-us/default.aspx

Important: The Microsoft Safety Scanner is a free downloadable security tool that provides on-demand scanning and helps remove viruses, spyware, and other malicious software. It works with your existing antivirus software.

Note: The Microsoft Safety Scanner expires 10 days after being downloaded. To rerun a scan with the latest anti-malware definitions, download and run the Microsoft Safety Scanner again.

Note: Any data files that are infected may only be cleaned by deleting the file entirely, which means there is a potential for data loss.

After Safety Scanner perform Windows Update.

Please follow the steps to install available Windows Updates.

1. Press Windows + R, type control.exe and then click on ok.

2. Change the view by (from windows explorer) to large icons or small icons.

3. Double Click on Windows Update.

4. Click Check for Updates. After the scan is complete, click Review optional updates.

5. Click the check box next to the updates, then click Install updates.

6. If prompted, review the license agreement, then click I Accept.

7. Follow the onscreen instructions to download and install the updates. If prompted, reboot your computer. 

Hope this information is helpful. Please feel free to reply, in case you face any other issues with Windows in future.

See if you can get rid of all the bits and pieces left on your machine from installing McAfee by using the Removal Tool from McAfee.

How to uninstall or reinstall supported McAfee products using the Consumer Products Removal tool (MCPR)
http://service.mcafee.com/FAQDocument.aspx?id=TS101331

Download the MCPR tool from: http://us.mcafee.com/apps/supporttools/mcpr/mcpr.asp and save it to a folder on your computer.

After you have run the tool (and reboted)  see if Defender will start.

https://www.google.com.au/search?q=windows+defender+0x80073afc&sourceid=ie7&rls=com.microsoft:en-AU:IE-Address&ie=&oe=&gfe_rd=cr&ei=xOtKVe2AFKLu8weLk4HwCg&gws_rd=ssl