You *could* build an XP machine for your data storage but sounds like you could make good use of a NAS (network Area Storage) device. There are some very high quality and reasonably priced pre-built devices available. Something like the
Synology Disk Station DS410J NAS Device would meet your needs. Please note that most NAS devices do not come with drives and must be purchased separately.
Another option, especially if you have extra or spare parts laying around is to build your own NAS. An excellent NAS operating system is
FreeNAS. FreeNAS is very stable and has any and all features you would need for a NAS. Best things about FreeNAS is that it is free to download and use, is open source with an excellent support community, and requires very little hardware to run efficiently and effectively.
As an idea, my FreeNAS box is running a Skt775 single core 1.6GHz Celeron 420, 1GB DDR2-533 (2x512MB DIMMS), on a Supermicro PDSME+ motherboard. I've got 5-640GB drive in a RAID5 on a 3Ware 8506 controller card offering 2.5TB of storage. It's housed in a cheapo Rosewill case and powered by a 450w Corsair PSU. It's like furniture, it just sits there and works.
Friend of mine went the pre-built NAS device (QNap 4-disk bays with iSCSI) for his home run business and it is a very impressive machine. But not having the money upfront, I went with the home built from spare parts route and snagged FreeNAS. He has since tinkered with FreeNAS in a VM and uses it to small extent to give his wife and kids access to music and movie files. After checking out both options, we both agree that whether a pre-built NAS device or a home built NAS, either will serve the purpose and are excellent, reasonably priced solutions. We brag to each other about who has the longest up time with me currently in the lead (over 5 months running stable without issues) because my friend recently had a power outage due to a storm in his area.