Excel 2003 SP3

Hope this is not too sketchy...
I need to review some data in an Excel spreadsheet that is setup somewhat
like this:
Column A = Date of measurement
Column B = Day of week measurement
Column C = 12AM Start of day measurement (like a temperature in degrees
Fahrenheit)
Column D = High during the day (measured)
Column E = Low during the day (measured)
Column F = 12AM end of day measurement
Here's what I need:
I'd like to setup a Column G, say, that would provide the average of ANY
interval I select (1 day, 2 day, 3 day, 4 day, 5 day, ....10 day etc.)
[Column G can be used for the Start of Day measurements, Column H could be
High measurement analysis etc.]
I'd like to be able to have an 'x' day analysis where 'x' is 1, 2, 3, 4 or 5
and then just review the data as it is calculated and based on the 'x'
selected,. Hence, a '5' could be the last 5 days of temperature averages and
10 would be the last 10 days of temperature averages.
So, how can I make the column variable? (I know how to do the division
here, it's the SUM() for whatever RANGE of days that I can't seem to know how
to do.)
TIA