When I was building my desktop PC, I omitted a DVD or Bluray drive, since I don’t have much use for such a device these days. I’m not sure whether it is possible to install Windows via USB, but there is a really simple method for Linux that can be performed even by users who have never seen a terminal window.
You require three ingredients: UNetbootin, an ISO file with a Linux installation, and a USB device. The application itself is self-explanatory. Here is a screenshot from the Mac OSX version I used:
You can even download a Linux ISO from within UNetbootin. Afterwards, you only have to select the USB device you want to use and click “OK”. To boot from USB you might have to change the boot order in the BIOS, though. My installation of Xubuntu went without any problem. Within about 15 minutes I was able to use my computer productively. There are quite a few needlessly complicated tutorials on booting from USB out there, making use of the command line tool “dd”, with which you might accidentally wipe our your hard drive, so you’re probably better off using a simple and straight-forward solution like UNetbootin.