No Optical Drive

Since you have no optical drive the only option appears to be USB.

Read these instructions carefully.

http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.7.pdf

From the manual:

You may be able to install Windows 7 with a USB flash drive that contains a Windows 7 ISO

image downloaded from Microsoft. To check whether you can do so, open Boot Camp

Assistant, click Continue, and see if the option “Create a Windows 7 install disk” is available.

To install Windows 7 with a USB flash drive, you need the following:

-An ISO image of Windows 7 Home Premium, Professional, or Ultimate downloaded

from Microsoft.

-A USB flash drive with 4 GB or more.

Boot Camp Assistant erases the flash drive, and copies to it the Windows 7 ISO

image and software drivers that let Windows work with your Mac hardware

Yes you download the .iso from Microsoft

http://en.wikipedia.org/wiki/ISO_image

Of interest regarding Win 8 and Parallels.

http://9to5mac.com/2012/10/25/parallels-warns-mac-users-to-not-upgrade-to-window s-8/

http://www.cultofmac.com/198145/mac-owners-should-not-upgrade-to-windows-8-just- yet-says-parallels/

FYI There is a community for the virtual machines like Parallels which might be a better place for support questions. I failed to note your interest in Parallels from your first post. My apologies.

https://discussions.apple.com/community/windows_software/windows_compatibility