It is possible, through bootcamp I believe. However Apple Recommends against it.
This is mainly due to the fact that if you completely override OS X, you also wipe out a lot
of Firmware information, which will mac the Apple created Windows Support Drivers unstable.
The best that I can recommend is to at least leave 10GBs of OS X left when creating the Partition,
and use the rest of the HDD space for Windows. Just to be on the safe side.