FreeNAS is a free Linux distribution (specifically is FreeBSD) with which you can transform an old PC into a network drive. For the numerous security features available to you this is more properly a Network Attached Storage (NAS) so it can be used in business.
FreeNAS supports the following file systems NTFS (using NTFS-3G), Ext2, Ext3, ZFS, FAT16, FAT32, UFS and UFS2. Are possible installations with iSCSI and RAID.
The advantage you have with this distribution (in addition to giving a second life to older PCs) is the ease of expansion of hardware features. For example, you can easily increase the capacity of the disks (by adding new hard disk on your PC) or speed of access to the LAN (replacing the 100 Mbit network card with a 1000-Mbit or more), or simply the ease of maintenance (if something fails it is easy to replace it, as they are all standard components and cheap).
Currently the version 8 is coming out. That promises further improvements! As soon as possible I will try to test it!
For years I use different PCs with different configurations transformed into FreeNAS, never having encountered any particular problems (in versions 6 and 7). In some cases they are "virtualized" or are virtual PCs (VMware FreeNAS).