When it comes to windows world we generally use VMWare/ MS Virtual PC for installing a virtual PC into our existing OS. QEMU is an open source software emulator which can be used on Linux distributions to install Windows OS. I understand that this has lot of features compared to other competitors. If you are using Linux as base OS and want to install Windows you can use this.
Here are few links related to QEMU.
Wikipedia Link.
Software distribution page.
Link Blog 
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