Acrobat Reader informed me that an update was available and asked if I wanted to install it.  Since PDFs have become a common vector for malware, I try to keep my copy of Acrobat Reader patched.  When installing the patch, however, I got the following error:

the installation source for this product is not available

If you google that error, you’ll find that it occurs a lot, usually because of a missing or corrupt installer (MSI file) or an erroneous registry key. Adding “Acrobat Reader” to the search string will turn up lots of problems and advice. Adobe has documented it here.  I tried all of their solutions, except for the Windows Installer Cleanup Utility, which is no longer available. Reinstalling, repairing and removing all resulted in the same error.

I then found this thread in their forum, which led me to looking at my registry.  “HKEY_CLASSES_ROOT\Installer\Products\68AB67CA7DA73301B7FF9A0300000010\SourceList\LastUsedSource” reflected an installer that did not exist.  I had a copy of the installer in another location, so I copied it to the location listed in the registry key, rebooted and tried again.  No luck.  I also tried editing the registry key to point to a different location (after backing up the registry branch, of course).  That didn’t work either.

What finally did the trick for me was deleting the registry key entirely, downloading the latest version from Adobe and reinstalling.  I don’t know what caused this problem, but it seems to be common.  I have several Windows updates that cannot be installed because of similar errors.  Malware checks have turned up nothing.

If anyone finds the cause of the invalid key, I’d like to hear about it.