Some form of Linux using a lightweight desktop environment like xfce or lxde would probably be your best bet if you want a modern operating system. If by 2x256mb ram you mean that your PC has 512mb total, then I would recommend you try out Xubuntu. If it has less than 512mb, try Lubuntu.
If you want to stick to Windows, the XP will probably be your best option. Just remember that it's no longer being updated, so it might be a bit of a security risk.