Hello LCP, and welcome to Sevenforums
While defender may be suffering through the same problem windows update is, they are not linked. Whenever you install a legitimate antivirus program, such as McAfee, Defender should
be turned off and disabled, as it is only spyware protection and nothing more. However, as I've seen numerous times, McAfee seems to cause too many problems to work around. Therefore, I recommend uninstalling it. If you would like a lightweight antivirus that is excellent at protection, try out Microsoft's Security Essentials: Spyware Protection | Free Spyware Protection | Microsoft Security Essentials
Now, to work out your problems. Even if you choose to stick with McAfee, please disable it, for now.
Download the file from Option 2 in Brink's tutorial here, save it to the desktop: Windows Update - Reset
- then run it. You should see a command prompt open and stream a long list of text
Follow Brink's SFC tutorial: SFC /SCANNOW Command - System File Checker
Restart and let me know if anything is different. Additionally, you may want to do a full scan with MSE (if you installed it) to check for any malware that may be part of the problem