This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Perhaps the old projects used an older version of jdk. Try to see that in the properties of the project, in the library section if the platform is configured right. Or drop the old platform configuration and use the default one.