XP because that's the disc I have in the house. The computer was gifted to me on the condition that I completely wipe the hard drive, and the original owner didn't have the OS disc to give me. Realistically, I'm going to be too busy with kids and other stuff to use Reaper to its full potential, but if I can record my drums and a few other things, and add some sequenced tracks once in awhile I'll be happy. I've got an M-Audio Delta 1010, which I've had for 12 years or so, as my ADC.