Raja's problem is replied to in first para itself. Rest of the post contains a few tips and summary that a newbie may be interested in.
Well this problem occurred good 6-7 months back. I remember installing an application after which display server started crashing. So I am quite at sea as to why this problem is occurring DURING installation. As far as I understand, you are not even able to see the OS running out of the CD. As Wim suggested, this may be due to a bad disk image. Alternatively, I think that Ubuntu is unable to correctly detect your graphics subsystem, because of which it is unable to launch the X server correctly. In this case, I would like to know your hardware configuration, particularly the motherboard you are using.
To be frank raja, i too regard myself a newbie. To get some confidence, you may like to go thru http://www.brunolinux.com/
for a quick overview. It actually has explanations to various Linux jargons. e.g. 'sudo' (Ubuntu/Debian equivalent of 'su', description available on the mentioned site) and 'console/terminal in Wim's post... get my drift?
The fourth question:
the dialog box appeared when i modified xorg.conf and rebooted the system.
Now you may ask how did I edit the file when the display wasn't even starting up?? Here goes... When this error appeared (Diaplay Server...) I pressed Ctrl+Alt+F2. that took me to a DOS like screen, called terminal or console in Linux lingo. (Actually there are 6 such terminals running side accessible by pressing Ctrl+Alt+F1 to Ctrl+Alt+F6, you can do this in any of these terminals) There i typed that Wim has mentioned: sudo nano /etc/X11/xorg.conf
. It then asked me for the administrator password that I set during installation, I entered it and the text editor (nano) opened with the file for me to edit. Edited and saved it and then rebooted the system.
Hope the post is detailed enough AND helpful.