Forgive me if the title is incorrect, but I'm unsure into which category my problem falls.

I have an Excel 2003 spreadsheet containing data which I wish to sum based on certain criteria. I hope the following conveys my reqirements adequately.

Col A contains different items as text. Column B contains a value related to these items. Column C is a running total based on values in Column B, whilst Column D also contains the calculated value in Col C minus an absolute value contained elsewhere in the spreadsheet.

I want to make the entries in Column D such that when a specific item of text appears in Col A, then the value in Column B __is__ subtracted from the previous entry in Col C, as normal, whilst the value in Column D remains as it was in the previous entry ie without the value in B subtracted.

I'm sure this is a fairly simple requirement, but am not familiar enough with Excel to sort it, and searching the Help section has proved somewhat daunting, and fruitless, largely due to ignorance on my part of what exactly to search for!