I experienced this problem using the graphical installer
(Using WinXP home. Command line installer worked fine)

C:\Documents and Settings\Gareth>java -jar "C:\Documents and Settings\Gareth\Desktop\jython_installer-2.5b3.jar"
Exception in thread "Thread-8" java.lang.NullPointerException
        at java.util.StringTokenizer.<init>(Unknown Source)
        at java.util.StringTokenizer.<init>(Unknown Source)
        at org.python.util.install.Installation.getJavaSpecificationVersion(Installation.java:112)
        at org.python.util.install.Installation.isValidJava(Installation.java:96)
        at org.python.util.install.OverviewPage.activate(OverviewPage.java:224)
        at org.python.util.install.AbstractWizardPage.doActivate(AbstractWizardPage.java:41)
        at org.python.util.install.AbstractWizard.showActivePage(AbstractWizardjava:414)
        at org.python.util.install.AbstractWizard.next(AbstractWizard.java:319)
        at org.python.util.install.AbstractWizard.gotoNextPage(AbstractWizard.java:215)
        at org.python.util.install.AbstractWizardValidator.fireValidationSucceeded(AbstractWizardValidator.java:78)
        at org.python.util.install.AbstractWizardValidator.access$100(AbstractWizardValidator.java:6)
        at org.python.util.install.AbstractWizardValidator$ValidatorThread.run(AbstractWizardValidator.java:16)