I cannot get my App Store to work.  It says "Failed to verify the preflight file. It is not signed by Apple." How do I resolve this?

When I upgraded to Mavericks the other day, I lost functionality of Mac App Store.  It worked fine in Mountain Lion.
Please help!
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Back up all data.
Launch the Keychain Access application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Keychain Access in the icon grid.
From the menu bar, select
Keychain Access ▹ Preferences ▹ Certificates
There are three menus in the window. Change the selection in the top two to Best attempt, and in the bottom one to  CRL. Log out, log back in, and test.