This actually sounds like the only functional JVM on the machine is the M$ one associated with the IE installation, which is crap anyway (missing and non-compliant classes). The association must have been changed by a later installation of IE after a SUN JVM was put on there.
I would uninst any SUN JVM, uninst V45, download the current J2SE JVM (either the compact JRE version, or the full blown JDK) and install it. Test out the JVM installation with one of the sample JAR apps, then create a directory for Mike's JAR. Drop the jar there. Right click on the JAR and try Open With... to see if it is properly associated with the JRE/JVM.
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One of the few remaining Mk1 owners...
#00015