Windows Vista BSNL Broadband VPN 5.0.00.0340 connectivity problem

Hi,

I have Windows Vista installed in my Toshiba laptop and BSNL broadband connection at home. I connect to the internet via wireless connectivity to BSNL supplied UT Starcom router-cum-modem. I also have Cisco VPN client version 5.0.00.0340 installed in my laptop. I have Symantec Antivirus installed on my laptop and Windows firewall running.

Problem: When I connect to my VPN server through the client on my laptop, it connects successfully. But within a minute the connection to the internet goes off. First it shows that no wireless connection, then it shows no interconnection at all.

Please help me find the solution to this strange problem.

Thanks
Amitabh