I've got a similar setup, the backups occur every 2 hours automatically, and it only copies files which have been altered.
I good explanation can be found here:
Essentially, rsync is installed on all machines (using cygwin on windows boxes) and you store config files saying which directories are allowed to exported to incoming rsync requests.
These are authenticated using passwords, so should be reasonably secure.
You then have rsync server (linux) with cron jobs running every xyz and new files are automatically copied across the network.
You can get a little more complicated and use compression etc, but I don't bother.
I also setup samba to share these backups based on username, so the users can retrieve files themselves if needed, you just map the network drive in windows (or create mount point in linux, if thats what they are using).
I used to use this web based software, which does the same thing, but I found it overkill eventually.
Might be worth looking at though:
Infact, its the already packages cygwin rsync daemon from backuppc which im using on the winxp machines:
http://sourceforge.net/project/showfile ... p_id=34854