output formula result into another cell

I need to create a formula that will write the result into another cell.

A1 sheet 1 is unprotected and contains no formula.
The user may write to this cell.

On sheet 2 in cell B1 is either a 1 or a 2.
On sheet 2 in cell C1 is a value.
On sheet 2 in cell D1 should be a formula that says if B1 sheet 2 is 1, A1 sheet 1 remains as written but IF B1sheet 2 is a 2, the value in C1 sheet 2 should be overwritten into A1 sheet 1.

I need A1 sheet 1 as unprotected because if B1 sheet 2=1, then the contents of A1 sheet 1 will be used in a different calculation.

I have written this as putting the result into a cell in another sheet because I usually protect and hide the calculation sheet in the final workbook.
But, if this is too complicated, then I can put the calculation and instruction into a remote cell on the same sheet and hide it.

It seems such a simple thing for Excel to write the results of the calculation in one cell into another cell and very useful, but it doesn't seem to be easy.

I am writing in Excel 2002, by the way.
Any assistance would be most happily received.

Results are written to the cell with the formula