XP all the way, I installed Vista 64bit on a spare drive on a system with 2.4 core duo + 4gb of ram and I had a terrible time with it, the churning of my HDD every time I rebooted my system, the million and one confirmations you get all the time just to install new software or change system settings, and networking, my god, what a pain in the ass, never in my life did I think it was possible for an os to reset my router randomly, and also the aero interface kept ramping up my GPU fan which I just couldn't believe, it's hardly a 3D os, I'd honestly prefer to use windows95 than Vista.