Excel averaging cell range

Hello,
I'm working in Excel 2007 and I need to calculate the average value of cells in column D whose values in column C are within a specific threhold. Right now, I enter =average(d2:d?) and visually scan column C to determine which row to end with (cells in column C whose value is less than 1). I then move to the next empty column and enter =average(d?:d??) looking at column C to now find the rows having values greater than 1 but less than 2. I repeat this several times until I have an average of column D data when column C values are greater than 5 and less than 6.

How can I automate this process? I have over 30,000 files that I need to process!

Thanks.