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I have done this before. When you have the .ISO file, use
Disk Utility to burn the .ISO to a DVD (Should say 4.7GB or more on it). Then run Boot Camp and follow the instructions and you should have Windows on your Mac.
When the Microsoft setup starts up make sure you format the BOOTCAMP portion of the harddrive, NOT the Mac OS X portion.
To boot into Windows, Hold the option key at Start Up, Select Windows, and Hit Enter.