How do I get OS X back on my computer?

I let my friend install Windows 7 on my computer that was running OS X 10.9. I told him to use BootCamp, but he said that he installed a OS X update, which ended up locking up my hard drive. Since he couldn't get into the Mac, he decided to erase all existing partitions, and format the main system to run Windows 7. Now the computer works fine as a PC, but I would also like to be able to dual boot as a Mac. Is there any way to create a disk partition on Windows, and install OS X as an additional opperating system. Any help would be appreciated.

*I also dont need the previous files on the Mac.