Actually after reading my problem, most of you will laugh at me. I myself am, for such a stupid mistake. What exactly happened was, I was downloading 32-bit version of Ubuntu 13.04 for my old PC. After downloading, one of my friend also wanted Ubuntu 13.04, although his PC had a 64-bit hardware. So I thought of downloading 64-bit version too. But mistakenly, instead of writing the ia64 version onto his USB drive, I wrote the 64-bit OS, replacing my 32-bit one. And I gave him a blank pen drive.
While installing Ubuntu, I certainly felt something was different. Different in the sense, that it was taking to much time to load. And then while using Gnome Tweak Tool, I found out it actually was 64 bit.
And I have also changed my HDD, so I don't have the ISO files any more. So any way to go to 32-bit without losing data or anything?