Customized buttons cause Excel shutdown in Vista?

[RAL/PSCN]

I'm migrating some large Excel 2000 and 2003 spreadsheets from XP to Vista
and the following occurs:

I can create the same macros I used in XP for these files and as long as I
run them by using "Tools", "Macro" "Run" commands from the toolbar, they work
fine.

However, if I try to assign a custom button to them or assign a "CTRL" + key
combination to run them, the next time I open the file and enable macros,
Excel shuts down.

Is this some peculiarity to Vista that anyone has encountered and can
explain a workaround for?

thanks,

roger