I am using a licenced version of CAD . Now I need to format my system..How can i install the same Licence after formating the system?  Is there any doing to do before formating the system?  Thanks In advance...

Just uninstall, reformat and then re-install. I recently did this when I upgraded to Windows 7. You're still installing the software on the same computer, so there shouldn't be an issue. You may need to contact Autodesk in order to obtain a new activation code though. I found that I couldn't re-use mine.