I would strongly recommend one of the Synology NAS devices. I use a DS-207+ (http://www.synology.com/enu/products/DS207+/index.php) with 2 x 500Gb drives in a RAID 1 configuration, it has been 100% reliable. Virtually silent device, very easy to configure (they just brought out a big upgrade to the web management interface) and configurable to do all the things you need to here.

You could re-use your existing hard drives (or just get 2 new ones, since they are so cheap now) and have the whole thing set up in a couple of hours. Far neater than dealing with a Windows-based system that needs regular maintenance, patches, etc. The only time I've rebooted the Synology device is when I've upgraded it for new functionality, which is an important difference. It regularly runs 3-4 months at a time without being touched.