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?

Does this help?

How to modify the number of undo levels in Excel (http://support.microsoft.com/?kbid=211922)

Hmm, check this folder:

C:\Documents and Settings\Jerry2\Application Data\Microsoft\Excel\XLSTART

Replace Jerry2 with your login account.

If there's anything in that folder, it autoloads when Excel starts. Try removing/renaming the files in here and see if the beahvior stops.

It could also be in:

C:\Program Files\Microsoft Office\Office11

Hmm, check this folder:

C:\Documents and Settings\Jerry2\Application Data\Microsoft\Excel\XLSTART

replace Jerry2 with your login account.

If there's anything in that folder, it autoloads when Excel starts. Try removing/renaming the files in here and see if the beahvior stops.

It could also be in:

C:\Program Files\Microsoft Office\Office11

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Thanks for this and your previous suggestion. Actually, I had already checked the two folders you mention: there were no files at all in either of them. About the other possibility - that add-ins are being loaded by instructions in the registry - I had also discovered this from Microsoft. But, never having had dealings with the registry before, I am reluctant (afraid even) to try this cure. For the present I'll just continue working in safe mode.

Robyg

For each version of Excel, use Regedit to go to:

(For Excel 2007):
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\E xcel\Options

(For Excel 2003):
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\E xcel\Options

(For Excel 2002):
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\E xcel\Options

Edit "UndoHistory" and change the value to between 1 - 100

Tratip, THANK YOU a million times. I was pulling my hair out trying to figure out why I can't redo. Your registry edit suggestion worked. I wonder if my registry cleaner program caused this problem to begin with... And if so, how to stop it from doing so in the future. Thanks again, you saved me hours..