Maven in Eclipse complains that "Unable to locate the Javac Compiler" whenever POM changed

I got error message every time I changed my pom.xml in eclipse.

Build errors for myapp; org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:2.0.2:compile (default-compile) on project web: Compilation failure
Unable to locate the Javac Compiler in:
  C:\Program Files (x86)\Java\jre6\..\lib\tools.jar
Please ensure you are using JDK 1.4 or above and
not a JRE (the com.sun.tools.javac.Main class is required).
In most cases you can change the location of your Java
installation by setting the JAVA_HOME environment variable

I have set JAVA_HOME environment variable. It is:JAVA_HOME=D:\Java\jdk1.6.0_23

Thanks.