I don't have a guess. I was hoping that Jim's suggestion would help.
Depending on your version of excel, maybe
Help|Detect and repair would help
Or uninstalling and reinstalling would help.
(As much as I dislike uninstalling/reinstalling, maybe it'll help...)
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And one pure guess that has no reason to work--but it won't take much time to
try:
Close Excel and
Windows Start Button|Run
excel /unregserver
then
Windows Start Button|Run
excel /regserver
The /unregserver & /regserver stuff resets the windows registry to excel's
factory defaults.
ps. There's a space between excel and the slash.
Dave you were correct. It did come up in safe mode but I get the same
results with tools->add-in. A warning and shut down. I had used Excel.exe
/s over and over again from the history. When I deleted that and retyped
then it came up in safe mode.