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You can't get it to boot via USB. Windows generally doesn't like to boot that way and Macs are particularly finicky about it to the point that the sanctioned USB installers don't usually work.
Three ways you can go that are proven:
1 burn the ISO to disc and boot from that
2 install it via parallels/VMWare fusion
3 create a throwaway partition on your internal HDD and install from that.
The latter two are detailed in a thread I will link to later. EDIT: here https://forums.macrumors.com/threads/601414/
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I'm not going to boot it on mac. I will boot it up on my PC.
How was I supposed to guess that?
The tools to do so on Windows are plentiful, if you don't already have Windows on your Mac download Virtualbox or a trial of Fusion/Parallels and install Windows from the ISO to that don't use the product key or activate it since you will only use it to build your USB.
I've personally used this guide to do so: http://www.intowindows.com/how-to-install-windows-7vista-from-usb-drive-detailed-100-working-guide/ or use the tool MS provides http://store.microsoft.com/help/iso-tool
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Another question, is it possible to install windows on boot camp using a bootable usb?
That's where we started. There are two methods that work on all Macs I mentioned above.
A USB drive prepared as in my last post seems to work for some folks on the 2010 MBA after they install rEFIt. http://refit.sf.net. See here: https://forums.macrumors.com/threads/1056451/ and http://www.tech-recipes.com/rx/9136...with-boot-camp-without-an-external-dvd-drive/
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So I can install Windows 7 through the Virtual machine and boot naturally? Will it come with any boot handler? Or boot loader to make it possible choosing os?
There is a built in boot loader in OS X as part of Boot Camp.
The pure VM route just uses the VM software to copy all the files from the ISO to the Boot Camp partition and then goes forward as a standard Boot Camp install. (I don't recall if there is an additional step required to make sure it will boot or not, but check the thread I linked earlier and ask specific questions there).
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