Not having a crystal ball, it is hard to know where to begin. What data do you have. How is the data arranged.
Is the data monthly, or yearly and are dates in a separate column. Are you trying to get annual averages, or monthly average sales prior to 2004, or do you want a single average for years prior to 2004.
Have you looked at the Help file for AVERAGEIF() in Excel 2007?
Have a look at it, try it, and if it isn't working, post your formula, along with information on the data.