Is it possible to covert a pc to a mac?

I have an Acer Aspire One, and a few older desktops. I was wondering if it is possible to run Mac OS instaid of XP. I really like the Mac system, and I don't want to buy another computer. If it is possible please tell me how I would go about converting it. I am willing to buy needed parts, and Whatever else is needed, but a free option is always better. Thanks!