Please tell Why m I getting this Error msg

I am new to java programming so please help me out with this

My java program is getting compiled but its not running.Everytime i run any program on my machine the same message appears-->

Exception in thread "main" java.lang.NoClassDefFoundError : classname

The same programs are running on other machines but not on mine.

I'm using j2sdk1.4.2_12

[380 byte] By [nakul.hotjavaa] at [2007-10-3 8:14:38]