Have you set a classpath in your computer? That is usually a mistake.
If you set the classpath yourself, record it somewhere and delete it completely.
If the classpath was set before you installed Java (which often means you have installed Quicktime), add the following to the start of the classpath:-
On windows:-On other OSs, eg Linux, Mac, Unix:-What is happening is that the Java runtime is looking in all sorts of folders for the file you want, and not in the . folder. If you get rid of the classpath, it sill use . as a default classpath.