There are several techniques to do this. The general thing however, is that you have to make the file available on the network, so that the linux server can get a hold of it.
This can be done in multiple ways, but the most obvious ones would be samba or FTP. I.e. share the folder on the windows box, and use smbmount to mount the shared directory on the linux machine. After this, you can use fopen() as you're used to.
Alternately, make the file available via FTP or HTTP on the windows machine, and fopen() it directly using one of these protocols.
