I see this all the time... Assuming your HDD is indeed a single partition formatted as Mac OS Extended (Journaled), you need to backup, do a clean install of OS X, restore your data from backup and then run the Boot Camp Assistant.
__________________ Apple Certified Macintosh Technician (ACMT) / Support Professional (ACSP) MacBook Air (13-inch, Mid 2012) 8GB RAM, 256GB Flash Storage Mac mini (Late 2012) 16GB RAM, Fusion Drive (128GB SSD/750GB 7200RPM) iPad mini 16GB, iPhone 4S 16GB
Try disk utility. It can repartition the hard drive without losing the original data.
Select the boot drive (the whole drive, the upper most choice in the list). Click "Partition". Make a new partition in the empty space at the "bottom" of the drive, where there is no Mac data.
I have a simpler method..
VM WARE FUSION
and then you have the best of both worlds no partitioning or worries.
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