that's an easy question: Windows is SO SLOW and bloated because users are stupid: People is educated to use scumm. They don't notice/care.
My XP for emulation purposes has only 6 processes running and it boots relatively fast: I am FORCED to use it because of WinUAE and SnesGT, but it's a lame system anyway.
Sooner or later, I'll run those two emulators on Mac OSX/self-compiled Linux with Wine/ CROSSOVER (reimplementations of teh windows API, no emulation/virtualization required) and I'll be free from Windows FOREVER.
A similar solution would be possible if AmigaOS/MorphOS ran on X86 hardware, and believe me there's silent, energy efficient, ergonomic X86 hardware (Asus EeeBox, not the same as EeePC, or Mac Mini X86). Screw those noisy towers...