Never "upgrade" to a beta OS. Dual Boot at minimum so your computer will still be bootable when shit hits the fan.Windows 7 64bit. Tempted to 'upgrade' to Windows 8 but I hate the metro / tablet-based style.
I have Windows 7 Home Premium 64-bit as well as a dual boot with 64-bit Ubuntu 11.10 (though I wish AMD would get off their asses with the GNOME 3 support for dual screens). If I just want to quickly browse the internet or just cba waiting for Windows to come up Linux is a welcome alternative. I also love the GNOME 3 interface I have installed on it with the lovely Docky for some real window management (I'm all for being minimalist but I need SOME form of quick window switching.)