Running total based on contents of a different cell

Hi again, I must say I'm learning heaps here, but this one has me stuck. Excel 2003 btw.

In the attached example I'd like to show a running total of length of a list of unit types. So column A would show the running total based on Column B (containing items typys A,B,C etc) Column C contains the individual length. So if Cell B23 contains an 'A' then cell A23 should total the rows in colum C that contain an 'A' in column B.

Gratefull as always for any help