This error occurs if the classes were compiled with a JDK that is newer than the one you're using to run them. Version 49.0 is the one generated by Java 1.5, so you're probably trying to run it with a Java 1.4 JVM.
Try compiling your code with the "-source 1.4 -target 1.4" options. This will only work if you use no Java 5-specific code, of course. If you do, upgrade the JVM on the other machine to Java 5.