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If you are using windows explorer you MUST turn off the feature that hides a file's extension.

Was the source file in the current directory when you used the javac command?
Use the dir command to list the files in the current directory to see what's there.

Sorry, I missed the JRE part. See Greg's post about what package the javac.exe file is part of. It's in the JDK not in the JRE.

The OS can not find the javac.exe file. The path to the bin folder that the javac.exe file is in must be in the PATH environment variable.