During the attempt to launch the VPN client, a message displays with the The necessary VPN sub-system is not available. You will not be able to make a connection error message.
This message can appear because of these three reasons:
The Service for VPN client is not started.
The VPN Client is not properly installed.
Firewall or antivirus programmes installed or running on PC while installing VPN client.
Complete these steps in order to resolve this issue:
Choose Start > Run. Type Services.msc.
Locate Cisco Systems ,Inc. VPN Service.
Double-click to open and start the service. From the dropdown list, choose Automatic and apply.
Try to launch the VPN client once the service is started.
In most cases, this should resolve the issue. If not, continue to step 5.
Uninstall the VPN Client and restart the PC.
Look in the Startup group for this shortcut: C:\Documents and Setting\All users\Start Menu\Programs\Startup\Cisco Systems VPN Client.lnk.
If the link exists, delete it. Also delete the VPN Client directory if it still exists.
Reboot the PC and reinstall the VPN Client, make sure there are no antivirus or personal firewall installed or running on PC while installing VPN client. Reinstall of VPN client should fully complete now.