Question: 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?

Answer: 
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](http://schoudhury.com/blog/articles/install-ubuntu-13-04-with-ubuntu-wubi-installer/)
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](http://www.omgubuntu.co.uk/2013/04/wubi-advice)