My friends say that when I die, my eulogy will be done in Excel since my entire life seems to revolve around it (you have no idea how many weird things I've done using Excel instead of, possibly, more appropriate tools - LOL!).
Anyway, Kurre's solution won't work because when cells are formatted as a number then no value is treated as zero (so every cell will become one whether you type a 0 in the cell or not). However, Des' solution will work for ya!
If you have any other Excel questions, feel free to PM me. ?