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: 
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)
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.