If you want the cell designation to stay the same when you copy and paste a formula from one cell to another, use an absolute reference. This means you need to add a dollar sign in front of each column letter and row number in the formula.

For more information, see Microsoft's Move or Copy a Formula in Excel page (http://office.microsoft.com/assistance/preview.aspx?AssetID=HP052014411033&CTT=4&Origin=CH010004901033).