When I try to compile a class in JBuilder, for example the "WelcomeFrame", I see this error:

"WelcomeFrame.java": Error #: 750 : initialization error: com.borland.compiler.symtab.LoadError: neither class nor source found for java.lang.Object"

What can I do?

This error indicates that you are using an invalid JDK in your project. Open the Project Properties and check the value for JDK under the Paths tab. If this is something like "unkown java", then you need to change it to point to a valid JDK (e.g. the JDK that was included with JBuilder 4, which will show as java 1.3.0-C).

To change/edit the JDK's available to JBuilder, open the Tools menu and select Configure JDK's. Click the help button on the Configure JDK's dialog for more information.