I'm not sure if Boot Camp can use the virtual disk that was created by // as the source of Windows, maybe someone will come along with that answer.
If the answer is No, then if you can find the original download of Parallels, you should be able to launch it and run the uninstaller.

Failing that, just delete the // application and the Windows File that was created when you first installed W7.

Also check in your Library/Application Data and your Library/Preferences for any reference to //

Then go ahead an use Boot Camp. It will create a partition on your HD and set you off on the W7 install process.
No, you will not need an external HD to backup files during partition. However, before you do anything drastic to a disk, you should ALWAYS back it up.