@sumitkumar,do u remember which version of ubuntu u have downloaded,i feel you installed a 64-bit version Linux on 32-bit hdd (i686).
this may be the problem and solution will be -download a version of ubuntu or Debian or any other distro U like -But make sure its iso name must contain something like i386 or i686 etc if u got a 64-bit processor and platform like AMD Athlon,download those iso ending with x86_64.64-bit distros support is not good in multimedia state.though u can use 32-bit versions on 64-bit it cant be vice versa with a x86 processor.