excel changing last number to zero

[connor66]

if i input more than 15 mumbers into a cell it always changes the numbers affter the 15th to a zero

ie 123456789123456789 would go to 123456789123456000

dose any one know why this happens?
i am using excel 2000, but it seems to hapen no matter what service pack is loaded.

 

[connor66]

done it formated the cells to text