Mac OSX will not work on a PC (well, not without some very significant technical workarounds). It requires an Apple Mac.

Linux would be a good place to start, though I understand the modern distributions often require quite hefty systems. There are members here that have a much more up-to-date knowledge that may help.

I hate to say it (being an Apple user ) but unless your parents are quite technically capable, your best bet is to stick with XP. It's familiar, and is relatively stable if protected properly. Maybe what you really need is better anti-virus/spyware/firewall protection? If that's what you mean by "getting dirty"?