System is Windows 7 Professional, 32-bit. When updating Windows Defender definitions, Windows Update constantly fails with error 0x80508001. I have tried updating using Windows Defender itself, but it fails with the same error. Manually updating using mpas-fe.exe does nothing, running as Administrator. This has been happening for several months, with the last working definition update on 24 November 2011. I don't really use Defender, but having it fail every time is getting annoying. Is there anything I can do other than reinstalling?

Hi, This issue occurred can be caused by third party software. You can try to install the updates again in Clean Boot environment or Safe Mode with Networking. How to boot in Clean Boot: http://support.microsoft.com/kb/929135/en-us How to boot in Safe Mode with Networking: http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode As you mentioned, you do not really use Windows Defender. If you have the other security programs installed, we can turn off Windows Defender.

I forgot to mention that I had tried to install mpas-fe.exe in Safe Mode, and it also failed. In any case, I just turned Windows Defender off since I am using other security software. Still, I wonder why Defender repeatly fails to update. IIRC, has happened before, in Spring of 2011, but it fixed it self. Oh, well.