Q: How to install ubuntu without a dvd? Recently I downloaded ubuntu 13.04 (iso). Now after extracting the iso file and running wubi - it says downloading required files though I had already downloaded the iso file (794 MB). So what to do now?

A: wubi in 13.04 is not in a releasable state. You have to install Ubuntu 12.10 through wubi and then upgrade to 13.04. follow this steps

A: Use the ISO that you have downloaded. You can make a boot-able pendrive by using this utility at http://www.pendrivelinux.com/. Then you can install ubuntu using this pendrive. 
PS. : in your boot options just check that USB is higher priority to the hard-drive.
Also, Wubi currently is not the best way of installing Ubuntu. You can read it for yourself here 

A: If you're on Windows, or even on Wine you could also try Unetbootin or LinuxLiveUsb.
I tried both of those for the first time yesterday, both were remarkably easy to use and did the job perfectly.

A: DO NOT extract ISO files. Instead mount it using ISO mounting program, and run Wubi. I suggest use Wubi only when you using Ubuntu 12.10

If you mean installing it on its own partition, instead use live-usb from pendrive using UUI (Universal USB Installer).
