"Install Now" button does nothing

Hello. At some point during the process of bringing a new installation of W2K
up to date, the "Install Now" button in Windows Update
(http://v4.windowsupdate.microsoft.com/en/default.asp) has stopped working.

I'm up to build 2195, SP4, I'm running Windows Update as an administrator,
and I have my firewall off. I have updates showing on the "Review and install
your selected updates" page (it doesn't matter which ones, critical or
otherwise), but when I click the "Install Now" button, nothing happens, no
error message, no page transition, nothing.

I have been downloading the updates from elsewhere (by searching on the KB
numbers), but there are some updates, such as the Root Certificates Update,
that are apparently only available through Windows Update. Apart from that,
it's a complete pain in the arse to track them down and apply them manually.

Any ideas? Thanks!