You could try the free version of Tversity on XP, that would work. I am not sure about XP sharing the Media Library out, or just making a network share off the NAS. Sorry I can’t be more help =( I don’t have an XP box here anymore…
Actually, I could setup a VM and test it for you, nerds love nerd work, ha ha! Check out Tversity in the meantime if you feel like it.
Also, here is what Microsoft says about sharing your media with Media Player, I am just not sure the XP version will see your ISO files =(