Easiest route is probably upgrading or re-installing the Java Runtime Environment (JRE).
- Open the Windows Explorer, from the Tools select ‘Folder Options…’
- Click the File Types tab, scroll down and select JAR File type.
- Press the Advanced button.
- In the Edit File Type dialog box, select open in Actions box and click Edit…
- Press the Browse button and navigate to the location the Java interpreter javaw.exe.
- In the Application used to perform action field, needs to display something similar to
C:\Program Files\Java\j2re1.4.2_04\bin\javaw.exe" -jar "%1" %(Note: the part starting with ‘javaw’ must be exactly like that; the other part of the path name can vary depending on which version of Java you’re using) then press the OK buttons until all the dialogs are closed.
Which was stolen from here: http://windowstipoftheday.blogspot.com/2005/10/setting-jar-file-association.html