Windows Home Server might be worth a go if you want a storage-type device, it's based on Server 2003 so is quite lightweight for an older machine, although it feels a bit like a poorly-finished modification of Server 2003 as opposed to a proper home server OS in its own right (eg the installation is long and involves a lot of reboots, it seemng to just install a regular Server 2003 and then install a whole load of extra packages that turn it into WHS, it has all the normal server admin tools installed but tells you not to use them as you can screw up the WHS management console's functionality that way, there's a mixture of WHS and Server 2003 branding in it eg the boot screen is the Server 2003 one) and there is this weird file corruption bug in it that hasn't been fixed yet, so I would be tempted just to go with a standard (2003 or 2008) server OS instead if I was making an older PC into a server.