This is a Registry error. Registry cleaners cause it. You do not have to reinstall autocad 2007. You can if you want but you only have a limited amout of times you can install or reinstall the program. It is better when you get this error to go into c:/programfiles/autocad 2007 and if the folders for the registry backup and open them and merge the backups back to you computer registry
Start with some of our most frequented solutions or visit the Installation and Licensing Forum to get help installing your software.