My two-cents...
Windows Home server license is about $150
Windows Server 2003 SB license about $360
Windows Server 2008 SB license about $670
Linux or BSD.........................$000
If you only want file storage,
FreeNAS is extremely small and fast.
If you think you may want more options, I'd say some flavor of Linux is your best bet. I'm partial to OpenSUSE
WWW Link. I haven't been able to get on the Ubuntu bandwagon (Ubuntu is going to change the world, cure cancer, and solve all our problems. I know, I read it on Digg so it must be true.

) That said, the ubuntu community has great support forums.
CentOS and Fedora are also excellent server distos.
I have a Mac G4 I picked up for almost nothing that I use as a web server. OS-X is BSD with the Quartz windows shell on top. It's solid and SAMBA or file sharing work great, but hardware can get expensive if you need to add or upgrade components.
Components: If you're going to store all you family files on this machine, I would build in some hardware fault tolerance. If you build from scratch, get a motherboard with at least 4 SATA headers, and RAID 5 support. If you want to add to an existing machine, find a SATA or IDE PCI card that can give you RAID 5 support.
If you have no fault tolerance, you will at some point lose all your valuable files. All hard drives fail at some point. If you lose all their files, your family will never trust one of your projects again.