Finding Val. in One Col. Conditional on Other

Hello,

I have a massive spreadsheet with 16,000 lines and I need to find the average of values in column D as long as the value in column C is the same. I am looking for a macro that averages values in column D until the value of column C changes and then loops to repeat averaging for the next set of values in D that have the same associated value in column C.

If anyone has a macro that performs this function that would be great. I can't possibly do it by hand and it doesn't seem that complicated, but I know nothing about macros.

Thank you.