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To delete the Windows partition and restore it to Mac partition, you don't need to do a complete format. You can do this without erasing Mac OS X. Follow the steps below;
First backup all your important files from your Windows side to an external hard drive as the following process will delete Windows and its files.
Then on your Mac, go to Applications folder, open Utilities folder and open Bootcamp Assistant. Click Continue on the initial Boot Camp Assistant Introduction window. Select Create or remove a Windows partition and then click Continue
Click the Restore button to remove the Windows partition and restore the disk to a single-partition OS X volume. That will do it.
If you have any difficulty on doing the steps above or if you want to completely delete both Mac and Windows and start fresh, please let me know.
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