From Microsoft Knowledge base article 211922
Close any programs that are running.
Click Start, click Run, type regedit in the Open box, and then click OK.
In Registry Editor, expand one of the following registry subkeys, as appropriate for the version of Excel that you are running:
Microsoft Office Excel 2007 HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Options Microsoft Office Excel 2003 HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel\Options Microsoft Excel 2002 HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Excel\Options Microsoft Excel 2000 HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Excel\Options
On the Edit menu, point to New, and then click DWORD Value. Select New Value #1, type UndoHistory, and then press ENTER.
On the Edit menu, click Modify.
In the Edit DWORD Value dialog box, click Decimal under Base. Type a value between 0 and 100 in the Value box, click OK, and then exit Registry Editor.
Start Excel. Excel stores an undo history for the number of actions that you specified in step 6.