如果安装了.Net 4.5，如何保持.Net 4.0的行为？

We have a Windows Form application that targets .Net Framework 4.0. After installing .Net Framework 4.5, the application starts crashing. We'll have to investigate the crashes and we'll most probably have to fix things on our side. But is there a setting we can turn on to keep the old behavior until we are ready to use .Net 4.5?We have a Windows Form application that targets