Java Programming

Hello
I'am trying compile and run simple java project with ANT.
Compilation is pass, but during run program on the console I'am getting this exception.

"java.lang.UnSupportedClassVersionError: Bad version number in class file"

I'am readed on this forum that:
"This exception can occur when the source is built targeting a JDK that is not supported by the JDK attempting to run it. In the above example, if the servlet was built targeting only JDK 1.5, but the web server runs JDK 1.4, the above exception will occur."

On my system(Windows XP) is instaled only JDK 1.5, i'am not instaled any other JVM.

What is the cause of this exception.

Please, Hepl me