So it sounds like you've been checking the Apple forums at their support site and haven't gotten much more than others saying "me, too." It's not a widespread problem, not that this will make you feel any better about it. Can you recall which Mac OS version were you on when this problem started? I know you were on Tiger when I saw you, especially since Leopard wasn't out yet.

I've encountered other Software Update issues but not this one exactly. Has anyone suggested going into Libary/Updates and deleting whatever's there? That's worked to fix some other issues, but those were with specific updates, not with all of them like your problem. It's worth trying that, then rebooting before you try again. And if that doesn't do it... well, if this were happening on my Mac, I'd go ahead and reinstall the OS. That may not be something you care to do, though.