Stuck in cell selection

When working within a file and selecting a cell I am immediately stuck selecting all cells. I understand it may have something to do with having EXT field at bottom RHS of excel. How can I set permanently to ADD (if this is the solution), any help appreciated as it is driving me nuts.....

hit f8 save file

Thanks but still same problem. Shift f8 changes Ext to Add but when I save file problem re occurs. Only way to close file is via task manager...

not shift f8 just f8
but try the repair excel
go to run(from the start button of windows)
put
Excel.exe /UnregServer
hit enter
then put

Excel.exe /regServer
hit enter
you will have to reactivate any add-ins tho