Hi, Ajeya. When it asks for username and password is it in graphical mode or already in text mode? Once you're logged in, you can try the command
this should start the X server and give the graphical environment, otherwise there is some problem with the drivers for your video card. Also, check in the file /etc/inittab the default runlevel: should be 5 for GUI, 3 for text mode. The relevant part is the line
immediately after the comments at the beginning.