SUMIF - Pulling Data Across From Different Spreadsheets

To comply with the rules, I have already posted this request in the Excel forums http://www.excelforum.com/forumdisplay.php?f=8. I have 6 spreadsheets that the team that I work for edits on a regular basis. I also have one summary spreadsheet that our director reads and uses for her reports. Most of the forumulae pulling information through are simple = ones, but there is one section which is a bit more complicated. The team have to list the number of client related activities they have done by month - eg:

A------- B--------C--------D---------- E-------F
Month--- Client--ITT-----Quotation-- Demo---Visit
1--------A--------1--------2-----------1
1--------B--------1----------------------------1
2--------A--------1--------1
2--------B----------------- 1-------------------2
3--------C------------------------------1

I am using a SUMIF to total the number of ITTs etc done per month: =SUMIF('S:InternalSales Figures2006-2007[ES 06-07 Spreadsheet.xls]Activity'!$A$3:$A$1001,1,'S:InternalSales Figures2006-2007[ES 06-07 Spreadsheet.xls]Activity'!$C$3:$C$1127)

This works fine if both the Summary Spreadsheet and ES 06-07 Spreadsheet are open at the same time, however, if ES 06-07 Spreadsheet is closed, then all I get in the Summary is #VALUE!. At the moment, all I can think of to rectify this is to do the SUMIF in each separate spreadsheet and then copy the information over to the summary one, which is duplicating information!