If you have anything on your computer that you wouldn’t want posted on Facebook, I can think of two options to keep yourself secure.
1) Upgrade to Win7 – Anything that can run XP can handle 7. Of course, with Microsoft pushing Win8, you might have a hard time finding a Win7 upgrade disc, and it’ll probably run about $100-ish.
2) Linux – Totally free, and simpler than many think. If you know what language you speak, what time zone you’re in, and can remember your name, you can install many Linux distros; quite a few are designed specifically for those of limited computer literacy. Unfortunately, even the slightest hint of xenophobia will paralyze you with fear, and blind you to the fact that Linux really isn’t any more complicated than Windows or OS X.