What release are you using? you should use the 1.0.beta10 because it improves the captive portal’s stability in presence of virus/worm in your LAN.
That is exactly the version I am using. There are no viruses nor worms as this behaviour does not depend on the OS on the clients: It is the same when using Linux on all the clients. (As zeroshell does not work reliably yet there is only one machine connected to each subnet for testing purposes.)