Go to Start-->Programs-->Cubase LE-->Application Data Folder

With Cubase closed, remove all the files and folders in here. Next, uninstall Cubase LE from Control Panel and re-install it from the cd. Make sure you have the latest updates for your sound card/interface.

Let me know if this helps you open a new project. If you can open new projects but existing projects still have the error then they are likely corrupt. In this case it might be needed that you copy the contents of the projects and paste them into new projects and somewhat take a fresh start on them.