I have Win Vista as operating system. My DVD Drive is not working. I want to replace the OS from Windows Vista to Ubuntu. Pla guide me by giving details of steps I have to follow
You can't use DVD or CD if you have that drive only, but using a pendrive is best option. TBH, rarely do people use DVD for installing Ubuntu. Most of them these days use USB. It's also lot faster and flexible installation that way.
You have to download the ISO file of the Ubuntu and then make a bootable USB drive.
There are tutorials on how to do it here:
Now, you have the option to dual boot along with Vista or remove it completely while you install Ubuntu. You can read about it here:
Now, while installing Ubuntu you can select options like Erase everything and install Ubuntu or Install alongside Windows Vista, to do what you want in an easier way.
DVD drive is not working, not a prob anyways DVDs are slower than USBs. So what you need to do is to install unetbootin in your computer (in Windows Vista). I am assuming that you already have a pendrive and you have downloaded an ISO image of latest Ubuntu 12.04, if not do that. Once done all you need to do is to run unetbootin, plug-in a pendrive and then start making a bootable USB/pendrive.
Now one more thing, if you are switching completely from Windows to Ubuntu, first thing I will recommend you to is get backup of all your important stuff. Because everything is going to be formatted. So get the backup then restart your computer, press F2 or F12 ( or whatever brings the boot menu), then reorder the bootmenu so that your pendrive boots ahead of your hard disk. It will give you the Ubuntu install screen and everything is a cakewalk after that. Still i would recommend you to get familiar with the install process looking at the screenshots provided at numerous guides across internet. You need to search like
One heads up: Do look at the net configuration guides as well. It's not same as Windows, so if you are a complete newbie this would really help you.