Today I was trying to apply the updates to my Windows 7 desktop. The update labeled “Definition Update for Windows Defender – KB915597 (Definition 1.101.1291.0) kept failing in the update process.

There really wasn’t anything helpful in the error messages to help diagnose what was going on. All I had was a “Code 8007051A Windows Update encountered an unknown error”.

It turns out that the Windows Defender service was not running on my computer. I went to the control panel to open Windows Defender and upon clicking the icon received the message “application not found”. Weird…something’s obviously screwed up here. I suspect I may have messed up some paths when migrating all of my programs to a newly added D: drive and changing the registry around so that new programs will automatically install to my d:\program files directory.

The fix ended up being fairly simple: continue reading…