If I open the application first, then do a File/Open from inside the application, everything is fine.
Other file types (PDF, ZIP) open just fine from Explorer.
The problem seems to go away by itself after a DAY or so.
Macafee Anti-virus and CA Pest Patrol (provided by Comcast) come up empty.
I found some articles about issues when opening through IE, or two copies opening or Excel opening, but not the workbook, but none seem to fit this problem.