I am using a licenced version of Auto Cad 2008.. Now i nedd to format my system .. What are the things i have to do before formating the system to reinstall the same licence  after system formation..   Please help me.....

Well, first off you want to back up all of your work, but that's a given.  Second, I would contact your Autodesk Reseller and check with them to make sure you go through the process correctly.