I would strongly recommend that you either disable Java entirely or upgrade to Snow Leopard (at least, if possible). While the latest malware threat (which employed a vulnerability in Java) has been fixed in recent systems, 10.5.x is too old (unless Oracle decides to put out a 10.5.x-compatible version on their own, which I doubt). The outdated Java you're running is still vulnerable, in short, and it would be best if you disabled it outright until you can upgrade.