Validation list filtering

Hi,

I am trying to figure out how to filter what is put into a validation list in my spreadsheet based on the input in a different column. It's easier to just explain what I actually have in my spreadsheet rather than come up with an example. I have a list of project IDs and their respective client IDs in two hidden columns. I have another column with each row being a pull down list of all of the project IDs. Next to that column the Client ID is entered. I need the project ID list to be filtered so that it only shows the project for the selected client. Theres probably an obvious solution I just dont know Excel that well.

Hi
Check this out:
http://www.contextures.com/xlDataVal02.html

JG