In my Excel 2003 the tab UNDO is permanently greyed out and inoperative (and ctrl z doesn't work either). I have uninstalled and reinstalled Office 2003 without getting any improvement. Also I have checked with Microsoft that all applicable updates have been installed.
I have discovered that if I start Excel in safe mode (by holding ctrl down while Excel opens) the problem doesn't occur - UNDO works okay. What can I do to get it to work correctly in normal mode?