How to install Windows 8 on a MacBook Pro with OS X Yosemite, using Boot Camp, without a USB drive or DVD
Installing Windows on a Mac via Boot Camp you only have two choices, DVD or USB.  You also need a Windows DVD (Full Install Media not Upgrade Media) or an ISO Image of the Windows DVD to create the USB Installer via the Boot Camp Assistant.

Yes, you'll be able to use the USB drive to store other files after the Boot Camp Assistant has written the files necessary to install Windows via USB.  However you need to first backup what you have on the USB drive as it will be removed in the process of creating the USB Windows Installer.


Note: For use in installing Windows the USB drive will be formatted FAT32 and as such you'll not be able to store files over 4GB in size for use with the "other files" you want to have on the USB drive.