i issued the following commands:
sudo add-apt-repository ppa:gnome3-team/gnome3
sudo apt-get install gnome-shell
and then i found the following change in the system :

but i dont find my desktop looking like this.
WHAT has gone wrong ? please help !
Looks like your theme has a problem. Found something similar on superuser.com here.
The answer from the source reads:
Try this, it worked for me:
sudo apt-get purge gnome-accessibility-themes sudo apt-get install gnome-themes-standardThen open the Gnome Tweak tool listed above and select Adwaita
Ubuntu 11.10 uses Gnome 3 by default, and you don't have to add any PPA to get that desktop. It's in the official repositories. It's called Gnome Shell.
It's perhaps unfortunate that you have installed Gnome-Shell on Natty (11.04) with that particular PPA.
In the PPA description:
=== Ubuntu 11.04 (Natty) === This PPA contains packages from GNOME 3.0 and their dependencies. It is considered EXPERIMENTAL and MAY BREAK YOUR SYSTEM. There is no downgrade process.
=== 2011/12/30 === There is no further Natty/11.04 support from now on!
The Gnome-3 packages in that PPA have now moved on and are increasingly incompatible with the remaining Natty packages. Even if you resolve this particular theming issue - you will increasingly encounter newer problems as time goes by and further updates are made.
I hate to say it - and the maintainers make it clear - you will need to reinstall your system :(
/home folder to an external hard-drive,