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You can only install Windows from a USB drive in one of these Macs:
- MacBook Pro with Retina display.
- MacBook Air.
- Mac mini (Mid 2010 and newer).
- iMac (Late 2012).
As you don't have any of those Macs, you have to install Windows through a DVD. Also, the only iMac that can install Windows using a USB drive only supports a 64-bit Windows 7 and 8 version with Boot Camp