Macro to deduct entered value

Hi,
Recently I made an Excel spreadsheet (Excel 2003) and used macros to add a specific amount to each cell when clicked. While setting up the spreadsheet, I had encountered another macro method that I am trying to do now.

The macro was simply, when a cell was selected and the macro button click, a small window would pop up (with user written description) and you input the amount you wanted to deduct, and the macro would do it. Now I can't find the website I learnt that from and I'm in real need of it. Would anyone be able supply the macro code for this function, or direct me to the right place?

Example, I click on a cell and then click the macro. A pop up window would appear, and the window has instructions that I created in the script editor ("Enter the amount you would like to deduct") and I would input the amount into the small box ("5"). The macro would then deduct 5 from the cell.

Thanks in advance.