I dual-boot Vista and Ubuntu 12.04( awaiting for the 12.10 ).
I use Vista because it was installed in my laptop in the first place but also because it's much more efficient with videos and games. The majority of PC games are written for Windows only for this reason.
Ubuntu I use for fast boot/reboot, convenient update mechanism, reliability and security( if you're no n00b ), and also because it contains all the necessary tools for developing my OS.
