If you have a network of windows clients and want to build a backup server with GNU/Linux, this 2 cents tip is for you. You need rsync, samba version 3, quota and cwrsync for windows clients.
First, the principles.
(*) Windows clients backup their files via rsync onto the backup server.
(*) If a user want to restore a file, his/her home directory can be browsed inside the network neighborhood as read only.
(*) Configure properly quota so the backup server won't run out of disk space.

On the server side, you need to install samba, winbind, quota, quotatool & rsync.

apt-get install samba winbind quota quotatool rsync

Then you configure you /etc/samba/smb.conf according to your network settings which is for samba and winbind while with /etc/rsync.conf, you don't need to change it. In /etc/samba/smb.conf, the following lines are essentials :

...
[homes]
   comment = Home Directories
   browseable = no
   writable = no
...

Seemlessly, these lines are available here and you just need to uncomment them. It's time to configure quota : You only need to edit /etc/fstab and, accordingly to your partitions (I put the homedir of the domain users under /home/DOMAIN and with its own partition) :

/dev/hda6   /home/DOMAIN   ext3   defaults,usrquota   0          0

This little bash script is helpfull when it's time to apply the quota rule for each domain user thanks to quotatool and wbinfo utilities :

#!/bin/sh
for i in `wbinfo -u`; do
    quotatool -u $i -bq 200M -l '250 Mb' /home/DOMAIN/
    # soft limit of 200 MBytes and hard limit of 250 MBytes
done

On the client side, we need to install cwrsync in a directory e.g C:\CWRSYNC. This batch can be scheduled to run once a day :

SET CWRSYNCHOME=C:\CWRSYNC
SET CYGWIN=NONTSEC
SET HOME=%HOMEDRIVE%%HOMEPATH%
SET PATH=%PATH%;%CWRSYNCHOME%
; The interresting thing begin here
rsync %USERNAME%@backup:
; To list the files on the server
rsync -auvzP /dir_to_backup/ %USERNAME%@backup:
; Copies the files in the directory onto the server
; If you want to assign a drive in the command above

Now, you can run this batch script to check evrything is okay.