When I burned ubuntu 12.04 LTS CD, I didn't check md5sum of iso file before it. I checked option check for defects and it ran perfectly. I doubt that Iso may be corrupted(Viruses,unofficial moded software etc). How can I check whether Live Ubuntu 12.04 LTS CD is having defects or not ?
Boot the ubuntu live-disk and click on Try Ubuntu option.Now you can check for errors without installing ubuntu on your hard-disk.
To check the md5sum of the downloaded ubuntu.iso file refer this.
This step can be done via a VM or by doing a restart of your PC.
Press ESC or an equivalent to force the PC to boot from the CD/DVD ROM media drive.
A menu will appear with options like "Try Ubuntu without installing". There should be an option for check disc for defects. Select this option to check your media for defects.
If you for some strange reason, do not see the check disc for defects option, the select F6 for other options, and see if you can find it there.