Problems connecting to Wi-Fi after changing ISP

Hi,
My Netbook has been having problems connecting to Wi Fi since I changed my ISP from O2 to Orange. When I change the Netgear routers security to “Disable” I can connect to Wi Fi but when the “mixed WPA-PSK+WPA2-PSK” option is chosen data won’t transfer even though a message says the network is connected.
My Netbook is an Acer Aspire and I’m using a Windows XP operating system. I have changed the router security settings and contacted my ISP’s help desk but neither has provided a solution.
Has someone had a similar problem?
Does anyone know what the problem is and how to fix it?
