My first boot device has 32bit Win7 RC. So I cant run the 64 bit upgrade from that platform. The second boot device has 32 bit XP pro which I also cannot run the 64bit u[grade from. And since I only have the Win7 upgrade, Im assuming I need a pre-installed OS? But since I cannot boot from the DVD to do a clean install, nor install from either 32bit OS, I'm also assuming it would be much easier for me to reformat the first, install the Vista 64bit that I have tha I never got around to installing and run the 64bit Win7 upgrade from there. So to answer your question, 2 os's. I didn't have to change the drive order. My first boot device is an HD, booting to Win7. Ill try and get a screen shot up soon.