Autonumbering based on cell input

This might seem like a simple request, but I'm still learning! :confused:

I have an Excel 2003 spreadsheet that I would like to set up with an autogenerated autonumbering system.

Column A is titled "ID#" and column B is "Type". Type is only ever "A" or "B" which the user selects from a drop down list. What I want is a system that will leave column A blank, unless the type is selected in column B, then once a type is selected an autogenerated increasing ID# is inserted into column A, in the form A-0001, B-0001, A-0002, A-0003, B-0002.... etc. And column A has to be locked from user input/edit.

Thanks in advance!!