sumif for values from Closed file

I am trying to summarise data from 50 different files (updated by 50 different users) into one file in one place. To do this I created a simple sumif function that links to those files. This sumif works correctly when those files are open. But when I open my output file after changes have been made to one or more of 50 input files, sumif function doesn't work when I hit update at the file opening pop up question and get #value error. If I start opening those files this error goes away and it works great again.

Does anyone know any work around for this or some other good way to do this in excel. I am using Excel 2002 on XP Professional.

I know ideal solution for this would be a database but given time, resource and cost contraints I can't do that for now. I guess I can bring all the data from 50 files into one file first and then do all my formulae but that would make my file size too big to be able to do anything.

Any help is highly appreciated.

Thank you,
Jay

SUMIF() doesn't work with closed workbooks... use SUMPRODUCT() instead.

so if the SUMIF() was =Sumif(A1:A100,"X",B1:B100) then the Sumproduct equivalent would be =Sumproduct(--(A1:A100="X"),B1:B100)