Hello excelhobbyist,
You can try using INDIRECT function, something like
=INDIRECT("'\\....\[week"&A2&".xls]'Sheet1'!$C$15")
obviously you need to include the full path.
Note: that INDIRECT won't work with closed workbooks though. If you
want it to work with a closed workbook you could try downloading
Morefunc add-in from this link
http://www.download.com/Morefunc/300...-10423160.html
and then use INDIRECT.EXT function