But are you sure that you are using Java 5 to run Tomcat?
In Eclipse, select the menu Window / Preferences, then select Server / Runtime Environments. Click on your Tomcat server and click the Edit... button. In the configuration window, you'll see a select box where you can choose the JRE to run Tomcat with. Select your Java 5 JRE.
If your Java 5 JRE isn't listed, then click on the button "Installed JREs..." to add your Java 5 JRE to the list first.