OK--that confirms a broken java install. Sure would be good to know how that happened.

Anyway, got word that your Java is missing a key file. To reinstall java, download from here

Apple - Support - Downloads - Java for Mac OS X 10.4 Release 5

You probably have to first delete the "receipt" that tells OSX your java is up to date first. The receipt is in the main Library (double-click the hard-drive to get at it) at /Library/Receipts/JavaForMacOSX10.4Release5.pkg

Hope that does the trick, because otherwise the only other step is the drastic reinstalling and updating of the system. Ugg.