http://java.sun.com/javase/downloads/index_jdk5.jsp
Is the link to the 1.5 download page. Go down and choose "JDK 5.0 Update 11" its the forth one down. Follow the directions to install it, then tell netbeans where to look. Also the latest release of Java is 1.6 (http://java.sun.com/javase/downloads/index.jsp), but beware many people arent using it yet so if you have programs complied in 1.6 they wont work if run in a 1.5 JVM.
Also, and I could be wrong, I think netbeans will defualt to a 1.4.2_11 environment, even if 1.5 is the version installed. You should check your version using the command line and running java -version
before spending all the time to download 1.5.