At this point your only real options are going to be a fresh OS reinstall. Since we can only communicate to this computer through USB that means we have 2 options. Remember any of these options will delete all data on the computer.
1. Use the recovery partition. When you turn it on you need to press a certain key to enter it. Keep repeatedly hitting CTRL+F11 as soon as you turn the computer on. If that does not work try F10, or F11. If that does not work, it may tell you on the bottom or top of the screen which key to press to enter recovery. If nothing works, I would call Dell because I cant imagine a computer with no cd or recovery partition.
2. Use a flash drive to install windows from. I have never done this before but its worth a shot and may be your only other option. Definitely call Dell first because I don't know how this would alter your warranty with them. But here is a place to start.
http://www.google.com/search?q=insta...ient=firefox-a
Hope that helps.