If you don't need to do math on it (i.e., it's a credit card number),
preformat the cell as Text or prepend an apostrophe (').
If you need more than 15 decimal digits of numerical precision, you'll
either need to use a different application, or load one of the add-ins
that claim to extend precision to 200 digits (do a Google search, there
are at least 2).
Hi
if you really need to calculate with theses numbers (e.g. they are not
credit card or telephone numbers) see for one add-in: http://www.big-numbers.com/