Tal
To do this you will need to change the registry setting;
From MS;
On the Start menu, click Run. Type regedit and click OK.
In the Registry Editor, expand the following key:
HKey_CURRENT_USER\Software\Microsoft\Office\8.0\Excel\Microsoft Excel
On the Edit menu, point to New and click DWORD Value. With "New Value #1" selected, 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 for Value. Click OK and quit the Registry Editor.
Note the more undos you enter the more memory that is required. Ms suggest max of 100.
HTH
Ivan