So this started with a Microsoft Security Essentials definitions update that wouldn't install, failing with code 8050A005. I did some research and uninstalled then reinstalled .Net4. This fixed the problem with installing Microsoft Security Essentials updates. Now all other updates are failing to install with either a code 643 or code 8007000d. I tried Microsoft Fixits 50123 and 971058, no change. Then I noticed I couldn't install an Adobe Reader update. I downloaded the update and when trying to run it I found the same 8007000d error in the log when trying to write a .cab file. I disabled Security Essentials. No change. Investigated the problem further, found that nothing will install. What's the next troubleshooting step?

Similar problem has been solve in MS Answers. Here is excerpt from answer: Description of the System Update Readiness Tool for Windows Vista, for Windows Server 2008, for Windows 7, and for Windows Server 2008 R2 http://support.microsoft.com/kb/947821 When you try to install an update for the .NET Framework 1.0, 1.1, 2.0, 3.0, or 3.5, you may receive Windows Update error code "0x643" or Windows Installer error code "1603" http://support.microsoft.com/kb/923100 I add: 1. Nothing starts itself and there may be a change of configuration, application or driver installation and similar changes in operating/application system that have influenced the behaviour. It is usefull to do analysis of what had been the cause. 2. install/uninstall problems : http://support.microsoft.com/kb/2438651/en-us 3. When automatic update does not work try dowload definitions and install the separately https://www.microsoft.com/security/portal/default.aspx 4. Where have you extracted the error code, from Event logs, WindowsUpdate.log or ReportingEvents.log? 5. The more tedious but exact method is analysis by Process Monitor (Sysinternals). Hope this will help Regards Milos

Adobe updates come in msp format that use windows installer (msiexec) to install them. Try re-registering msiexec Click Start, click Run, type MSIEXEC /UNREGISTER, and then click OK. Even if you do this correctly, it may look like nothing occurs.Click Start, click Run, type MSIEXEC /REGSERVER, and then click OK. Even if you do this correctly, it may look like nothing occurs, or you may briefly see an hourglass. After you run this command, the operation is complete.Try your Windows Installer-based application again.