UNDO doesn't work

Hi!!

I just purchased and installed Office 2010 on my lap top. I am a 2003 user trying to set-up and learn 2010 and I cannot get my UNDO/REDO buttons to work.
They are activated while I am editing a cell but as soon as I press enter they dim and become inactive and I cannot UNDO the last or any action!!!
HOW DO I TURN ON THIS FEATURE?
Any suggestions would be greatly appreciated by this 2010 novice!!
THANKS!!!

UPDATE:
If I start excel in safe mode the problem goes away and Undo/Redo works just fine. I have all add-ins disabled and when I look in VBA there are no macros visible!!!

afaik this is causes by code, the undo stack is deleted.there may be some code in your personal.xls

If it's a new install & you haven't recorded any macros then you probably haven't got a PERSONAL.xlsm.

If you do have a PERSONAL>xlsm then it will be hidden. To unhide a workbook select the View tab and click Unhide.

Screenshot