I am using eclipse to compile a java application.

It works fine if I right click the class that contains main. But If I run it
in dos window, I get this error message,

Exception in Thread "main" java.lang.NoClassDefFoundError:

I had this error message a few days ago, then I fixed it, and then I havn't
gone back to command line util today I finished everything and it works fine
if I just right click and Run As..Java application.

When finally I returned to command prompt, the error message came back.

I tried previous version of my application and the same error message. It
doesn't seem to be a problem with my application but I do remember when last
time the error came, I fixed it by going to the proper path.

Now, no matter what I try, I fail.

Anybody can help? I am using windows xp sp2 with the latest eclipse.