You can use Time Machine for Apple after setting up a share (for more info, see http://doc.freenas.org/index.php/Apple_(AFP)_Shares).
For Windows, using Win7, the built in backup supports using a network machine only in the Pro OS versions. You can set up a CIFS share and backup directly to the share with Win Pro. For Home versions, you can backup to a local drive then use a scheduled batch file to copy the bu directory to your share. Or you can look at 3rd party bu software options.
Just so you know, Time Machine will suck all the space it can get its grubby little hands on. Having a separate volume with a quota for TM is probably a good idea.