Windows 7, the last great OS from Windows. XP was the most stable though. I might switch to a Mac with dual boot though. I just can't get over the price, for $800 I got an AMD 6 core cpu, 8gb of ram, 1TB HD, 750W power supply, and a graphics card which is not outdated, I believe GTX 460? Renders full HD video pretty quickly but I need to upgrade that. I also got a new case and added a liquid cpu cooler, corsair H60 that I got from Fry's on sale for $30.