1. Upgrade the repositories list first. Run this in terminal (you need a working internet connection) "sudo apt-get update" without quotes
2. Goto synaptic and search "compiz".. Mark for installation and Apply.. It will download from the repositories and install automatically..
3. Then goto appearances (right click on desktop and select "Appearances and Preferences") --> visual effects tab. Select the last option. If your hardware/driver supports 3d desktop effects, you will be able to select the last option. Else you will be stuck at no effects and basic effects
4. Alt + F2 to bring the run window and run compiz --replace
5. Again do Alt + F2 and run "ccsm" command to bring up compiz options
6. Select 3d windows plugin and other plugins if you want
7. Goto general tab and choose 4 desktops ( default is 2.. increase the slider to 4)
8. Close the compiz manager
9. Press Ctrl + alt + mouse left button and drag the mouse.. Voila you have a 3d desktop
.. Enjoy your computer..!!