Calculating mulitple workbooks

[nichiaichi]

I am new to the forum and not a pro with Excel but I am some what familiar with it's capability. I use excel to calculate cost and profit with my business and now would like to use to use Excel to monitor my inventory. Here's my situation on my computer:

Folder: Order2010/AprilOrder.xls
Folder: Order2010/MarchOrder.xls
Folder: Order2010/FebOrder.xls
Folder: Forms_Inventory/productList.xls

Each of these WORKBOOKS have many worksheets. Column A of my productList.xls is the product ID (serial number). Column C is quantity remaining of the item. I would like this column to be calculated automatically when other worksheets are added.

Order2010 Workbook (example AprilOrder.xls) is a bookwork with many sheets. Each sheet represent a customer and what they order. In each sheet can contain any number of a product (represented by ProductList.xls, column A). In column F of a customer order is the total quantity of her order which I would like to automatically sum up and recorded. The dilemma is that each customer can order any amount from a list of over 1000 different items. Is there a method I can automatically sum up a particular product ID in the entire workbook?

Thanks for any suggestions.