Setup secure file transfer environment

I have three categories of file to be transfered over a WAN in a secure but effient way. The files are quite large and include both txt and spool files. The first set of transfers are from one location to multible branch sites. The other one is frommultible sites to the same location. This is to be done daily and the window of transfer time something like three hours only. The environment is preferred to be Windows. Your suggestions please.