I'd say XP, but with themes disabled. Themes tend to bog down the slower systems slightly (though, to be honest, I don't know how much it'd affect a 800 mHz system). I'd also install the nVidia drivers for the graphics card rather than rely on the stock XP drivers. A good IE alternative (Opera or Firefox), a good antivirus (I'd recommend NOD32 if the people are willing to pay a subscription). I would personally uninstall Outlook Express, though that really depends on personal preference. Of course, completely patch the box and disable services that aren't really needed.

Cheers,
cgkanchi