It means that the Java runtime (the "JVM") can't find a particular class; usually this message ends with the name of the missing class. Now, what do you mean here by "boot up?" Where do you see the message?
If u r getting this error while excuting java program, then there must something wrong in your jdk classpath. U check up ur classpath and then try it. And what do u mean by boot up, u shouldn't get such kind of error while starting the machine.