Thanks for your prompt reply. Most of the syptoms that I experienced were described in the first post. The adapter that I was connecting to, and attempting to firewall, was a TAP-Win32 adapter installed by OpenVPN 2.0rc6.
After seeing your reply I checked the registry. That key was already gone (NeT Filter was already uninstalled), like you expected. So, I uninstalled OpenVPN 2.0rc6, and installed the latest version, 2.0rc16. That didn’t seem to fix the problem, so I went into the TCP/IP properties for the new TAP-Win32 adapter, and then I enabled TCP/IP Filtering for all adapters, and I permitted all for TCP, UDP, and IP. Apparently, that cleared the problem, because a remote client can now connect to the VPN.