This is perhaps the most useful feature of FreeNAS to the average home user.
Depending on the computers you use, the File Server could use CIFS/SMB or NFS to serve files.
CIFS/SMB is universally supported by Windows, OSX and Linux, so is perhaps the most useful. Hence it is what I have enabled.
To do this: Go to Services -> CIFS/SMB
Tick Enable near the top right.
All the defaults are generally OK, although I have enabled Large read/write, Use sendfile, Store DOS attributes and Null passwords in the Advanced Settings.
If you have a Windows XP network already, the Workgroup name may be MSHOME or WORKGROUP. You can change it.
Click Save and Restart near the bottom.
Then go to the Shares tab.
Add your mounted share here by clicking on the + sign at the bottom right of the table.
Enter a share name in the Name box, and then click on the … to the right of Path to select the appropriate directory. You can select the HD you have formatted; it should be /mnt/name_of_HD (where name_of_HD is the sharename you gave to the HD when you selected a MountPoint.
Click Add near the bottom.
You should now be able to find this HD from any computer on the same Windows (SMB) network; the default settings allow read and write access for anyone.