ClassNotFound problem in startup times

When tomcat starts,
java.lang.ClassNotFoundException is reported in stderr.log: org.apache.catalina.startup.Catalina
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged( Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
at org .apache.catalina.startup.Bootstrap.init(Bootstrap.java:221)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:409)

Judging from the exception, the java class is not found, where is it? Is there a problem with the settings? Thank you~

My settings are like this 1. The
jre folder is placed in D:\myserver\runtime\jdk1.6.0_21
  tomcat is placed in D:\myserver\runtime\Tomcat
2. Set up in the batch process of installing the service
set JAVA_HOME=D:\myserver\runtime\jdk1.6.0_21
and call service install myserver
3. in D:\myserver\runtime\Tomcat\bin
set JRE_HOME=D:\myserver\runtime\jdk1.6.0_21\jre
nothing else is set, thank you~