First, if you want to share files hosted on GNU/Linux machine with Win**** clients, I don't think you have the choice;
the only solution I know is Samba (http://www.samba.org/
It is easy to setup for simple shares.
Then, I would suggest BackupPC (http://backuppc.sourceforge.net/
) as backup solution (because I use it at work and I am rather satisfied) to back up Windows hosts. It provides a web interface to manage backups and restores.
There are a lot of backup solution (Bacula, Amanda, etc.) but I don't know if they are multiplatform.