Are there any environment variables that can help me diagnose problems with my JDK/JRE configuration?

Yes, When set to any value, the _JAVA_LAUNCHER_DEBUG environment variable causes the JRE to generate numerous debug messages at startup--often useful in resolving configuration problems. Here's a sample:

Microsoft Windows 2000 [Version 5.00.2195]
(C) Copyright 1985-2000 Microsoft Corp.

C:>java -version
java version "1.3.1-beta"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1-beta-b15)
Java HotSpot(TM) Client VM (build 1.3.1beta-b15, mixed mode)

C:>set _JAVA_LAUNCHER_DEBUG=1

C:>java HelloWorld
----_JAVA_LAUNCHER_DEBUG----
Version major.minor.micro = 1.3.1
JRE path is C:Program FilesJavaSoftJRE1.3
jvm.cfg[0] = ->-hotspot<-
jvm.cfg[1] = ->-server<-
jvm.cfg[2] = ->-classic<-
3245 micro seconds to parse jvm.cfg
JVM path is C:Program FilesJavaSoftJRE1.3binhotspotjvm.dll
JavaVM args:
    version 0x00010002, ignoreUnrecognized is JNI_FALSE, nOptions is 1
    option[ 0] = '-Djava.class.path=.'
94052 micro seconds to InitializeJVM
Main-Class is 'HelloWorld'
Apps' argc is 0
4030 micro seconds to load main class
----_JAVA_LAUNCHER_DEBUG----
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld
C:>