I know this operating system is more than 10 years old, but I heard that there are still some hard core followers. OzGameShop has just dropped the price of Windows XP SP3 Home Edition OEM to $29.99 delivered.
Besides, it can be a cheap way to upgrade to Windows 8 Pro later ($40 before 31 Jan 2013 if I remembered correctly).
Probably my favourite OS of the past 15 years. I think I still have the original retail box I bought in 2002 floating around here somewhere..