CMD not recognizing javac.exe

I'm surprised that it worked before - had you cd'd into %JAVA_HOME%?

If you want the programs in the bin folder to be accessible from anywhere in a command prompt, you need to add it to your PATH. Since you don't have a user-specific PATH variable yet, create one, and set its value to C:Program FilesJavajdk1.7.0_79bin. Then you'll be able to enter javac on a command prompt anywhere and it will work.

Also, correct values of the JAVA_HOME variable do not end with bin. (Source.) That may affect other programs that try to use Java.