The correct version of Java Runtime Environment (read Download Page) must be installed at the computer. Check JRE version.

More information:
• Open the Add/Remove programs window this way: Start > Settings > Control Panel > Add/Remove programs
• A list similar to this will appear:

If no JRE is installed, install JRE.
If more versions of JRE are installed: Uninstall all versions and install newest JRE version.
If an old version of JRE (less than 6) is installed: Uninstall the old version and install newest JRE version.
If version 6 of JRE and no other versions are installed: Your installation is OK.

Get JRE

Make sure you install:
• the 32 bit JRE version at 32 bit Windows systems - (at the JRE download page called: "Windows x86 Online" or "Windows x86 Offline")
• the 64 bit JRE version at 64 bit Windows systems - (at the JRE download page called: "Windows x64")

The Classpath variable is used by Windows to tell the Java Runtime Environment(JRE) in which folder(s) to look for class files (Java executables).

If the Classpath variable is set to "C:\Program Files\SomeApplication" the JRE will look for the class file in this folder.

Classpath variables can be combined to cover multiple folders like this: "C:\Program Files\SomeApplication;C:\Program Files\AnotherApplication".
Note: The semicolon delimiters multiple folder paths.

To force the JRE to look into the current (active) folder add ".;" in front of a Classpath variable like this: ".;C:\Program Files\SomeApplication".
Note: The period is shorthand for current folder.

Some installation programs writes/modifies the classpath variable incorrectly.
Mostly these programs define their own folder only - and forgets the ".;" as the first folder path in the classpath line.

Omitting ".;" will force JRE to look for class files in the program's own folder exclusively - and NOT in the current folder.
I think you understand why this is a very, very bad habit: Windows will tell the JRE to look for class files in the program's folder only and will never look for class files in the current folder.

Note: At many computers the Classpath variable is not set at all. This will work like the period (current folder).
 

Editing the Classpath variable in Windows 2000 and XP
• Go to the Desktop
• Right click My Computer
• Select Properties
• Select the Advanced tab
• Click button Environment Variables
• Find Classpath in the System Variables window
• If Classpath is present highlight this and click Edit
• Add a period and a semicolon in front of the line as shown in the blue line in the image
• Keep clicking OK until all windows are closed again

Editing the Classpath variable in Windows Vista, 7 and 8
• Go to Control Panel
• Select System and Maintenance
• Select System
• Select Advanced system settings
• Select the Advanced tab
• Click button Environment Variables
• Find Classpath in the System Variables window
• If Classpath is present highlight this and click Edit
• Add a period and a semicolon in front of the line as shown in the blue line in the image
• Keep clicking OK until all windows are closed again

The Path variable is used by Windows to tell Windows where to look for the Java Runtime Environment (JRE). The JRE file name is javaw.exe.

If for some reason this file is not found in one of the folders listed in the Path variable; the JRE will not be found; will not be started; and Java-based applications can not run.

Normally the javaw.exe file is stored within the folder "C:\windows\system32"; which normally is the first folder name in the Path variable.

If javaw.exe is NOT in this folder, the easiest method is to locate "javaw.exe" in the file system; CREATE A COPY of this file; and place it in the "C:\windows\system32" folder.

A more difficult and more risky operation is to add the name of the folder where javaw.exe is stored (e.g. "C:\java\bin") to the Path variable.

Read in the section above how to view and edit System variables.

If you edit the Path variable remember to type a semicolon between folder names.

A programming error will create a Java Runtime error and will stop the program execution.

When this happens nothing else but closing the program can be performed.