Java is needed to run the application. Please make sure that you have current java installed. Below is a quote from Pyru detailing easy procedures to get your client running:

You need to ensure you have Java 8 installed. To find out which version of Java you currently have installed, please click on "Start", type "cmd" and press enter. Once the window is open, please type "java -version", press enter and note the output. If the out put is not "java version 1.8.*.*", you will need to download and install, Java 8 from here.

Once you've installed java 8, please also run "jarfix", which can be downloaded from here.

Note: If you already have Java version 8 installed, just run "jarfix", and it should resolve the issue you are having.

Hope this helped!

It seems your file type associations are buggered. What's happening is your default program to open "jar" files is no longer set to java, rather an archiving program, such as WinRAR or WinZip.  You will need to install Java 8 and run the "jarfix" program which has been included in the tutorials linked to you.

Thanks,
Pyru