Conditional Formatting (> 3 conditions)

Hello,
I am creating a grade book in EXCEL (Office 2003) and need to have more than three conditions for colouring the cells.
Very simply, I have x number of rows each starting with an 'aspirational grade' in the first cell followed by the marks for around 25 pieces of work.
The aspirational grades are A*, A,B,C,D, and E.

If the AG is 'A*' then the cell containing a mark => 90 will be coloured green, =>80 and =<89 - orange, <80 red.

However, if the AG is 'A' then the cell containing a mark => 90 will be coloured yellow, =>80 and =<89 - green, =>70 and =<79 - orange, and <70 - red.

If the AG is 'B' then the cell containing a mark => 80 will be coloured yellow, =>70 and =<79 - green, =>60 and =<69 - orange, and <60 - red.

If the AG is 'C' then the cell containing a mark => 70 will be coloured yellow, =>60 and =<69 - green, =>50 and =<59 - orange, and <50 - red.

If the AG is 'D' then the cell containing a mark => 60 will be coloured yellow, =>50 and =<59 - green, =>40 and =<49 - orange, and <40 - red.

If the AG is 'E' then the cell containing a mark => 50 will be coloured yellow, =>40 and =<49 - green, =>30 and =<39 - orange, and <30 - red.


In all cases, a cell containing 'A' (for absent) will be coloured grey with white text and one containing 'NW' (for no work) will be coloured brown with white text.

Each change of colour should take effect on entering the results.

If you could set me on the right VBA track, I would be most grateful.
Thanks in advance
RG