So I just bought a new MIM Stratocaster today and when I started playing it at home I realized the D and B strings buzz when I play them open (not when they're fretted). I restrung the B string just in case that was all it needed, but it continues to buzz. Can anyone tell me what the problem could be and how I can fix it?
Nut slots could be cut too deep, neck may need more relief, or action needs to be raised a bit.
You should always try to learn to do a basic setup of the guitar. It helps to know what a proper setup looks and feel like. Take a look at this video for action adjusting. It covers pretty much all the basics for a stratocaster. http://www.youtube.com/watch?v=27grJnhqsEY

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Take it into good shop for a setup. It could several things, action, truss rod, A fret could be high, the nut issue mentioned above etc... If you don't know how do a setup, it's worth the extra $$ to have it done by a pro.