One thing I did with my old XP box that refused to die, before I finally upgraded it to modern times, was loading it up with the biggest old hard drives I had laying around, stuck it in a corner down in the basement, attached it via Ethernet to my local network, shared all those drives and basically turned it into a NAS (network attached storage). I then used it as my backup server, saving backups of all my other computers to this old XP machine.
I just made sure I went into my router's admin menu and blocked all incoming and outgoing traffic from the Internet to that machine. This is often easily accomplished via the Parental Controls feature in the router.
But I still think turning your into a Linux box is a good idea. It is surprising, with all the free applications out there, how capable a Linux box can be. There are many who have totally converted to Linux from Windows and never look back.
