I will be honest Windows XP SP3 + some personal fiddling for better protection (along common sense) on a really really weak desktop, with certain parts so old you cannot even change GPU without changing everything else (GPU not recognized by Windows 7).
I wanted to get better PC nowadays, but I got actually worried how high prices for components went yet I am not up to invest into rig that is already obsolete by modern stardards. Especially if you can't even afford such a waste of money.
I have a laptop with Windows 7, but vanillia Freelancer runs with around 15-20 fps due to lack of dedicated card.