Windows Task Scheduler suddenly empty

My Windows 7 Task scheduler has nearly 100 tasks scheduled (most by Windows and HP). Suddenly it can't read most of its own data and therefore can load only a dozen tasks for viewing or editing. Scheduler load errors read "Reading data failed" and "File does not exist". The task job files on my system are stored in C:\Windows\System32\Tasks where I found 103 files in 6 folders:

Most of the files in this directory tree are XML files with no extensions
Many are byte for byte duplicates of the XML files, but have the extension .BAK
Some are XML files with extension .JOB
Some are binary with no extension
Some are binary with extension .JOB

I restored the "Tasks" directory (and all of its subdirs and files) from an early September backup (when Task Scheduler was working normally) but now NO tasks shows up in Task Scheduler. Strangely, when I view Scheduled Tasks in Microsoft Autoruns, it lists almost 100 tasks when it's first run. Some can be toggled on and off but most elicit an error message stating that the file cannot be found. On refresh, all but the same dozen third party entries disappear from AutoRuns window.

I am running Windows 7 Premium Home Edition (sp1) on a Hewlett-Packard PC with plenty of free disk space (Total 900 GB, Used 65 GB), 2.90 GHz ADM Phenom II X4 840T Processor, and a Hitachi CD/DVD drive.

Any thoughts on how to tackle this problem?