I use WinXP most of the time, becasue it is a stable system with a lot of functionality and because i'm running visual studio on it. And because there is a lot of high quality application for this system.
I also use Unix(R) like system. But I don't use Linux. I use FreeBSD instead. In my opinion, FreeBSD is much better designed than linux and it is (i know i shouldn't say it) a real unix(R). I can run any linux application on FreeBSD if i need since there is Linux Binary Compatibility.