Hello,
It is usually recommended to have Windows installed on Boot Camp and to create Parallels Virtual Machine which will load this Windows directly from Mac, so you would not need to reboot the whole computer to access Windows partition.
It is better because of several reasons - first of all, you will be able to run several applications which are not supported by virtual machines. Also, the performance is better (with VM under Boot Camp), as you operate with a real hardware (at least one emulation level is excluded, so less resources are needed) ect.
Moreover, you would not need to reinstall anything on Windows if the VM is occasionally corrupted. When starting your virtual machine you will have exactly the same configuration as you have in your BootCamp.
Please, find more information about creating a BootCamp-based virtual machine in Parallels Desktop User Guide:
http://www.parallels.com/download/file/doc/Parallels_Desktop_for_Mac_User_Guide.pdf
on the page 20 and instructions for creating such a machine starting from the page 266.
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