Got the same problem, tried switch to maximum perfomance in batter options, installing srs sandbox audio 1.9, realtek drivers
2.50, 2.20, 2.19, 2.18 and Windows default drivers. Laptop Asus x50sl. With Vista32drivers sound crackle becomed less noisy but still annoying. I dont have any boost or CnQ in bios.
Crackle increases when i'm speaking on skype + winamp playing.