Preventing Cells from Changing Format

I am working with a large workbook and sometime, when I open it, it changes the majority of cells to a date format. Is there an easy way to prevent this?

Bump...any help would be greatly appreciated!

Can't really help here, but a route you may want to consider - I feel I have heard something way back - is to do with Options when 'saving' the workbook.

Hope I don't waste your time.