From page 21:
There are three ways to establish a VPN:
• You may be able to log on to the VPN on the Windows logon screen. This way, the computer
can connect all internal network resources (such as network drives) immediately when it
starts up. See Connecting to a Gateway at Windows Logon below.
• You can manually connect your IPsec VPN Client to a gateway and end-point of your choice.
See Connecting to a Gateway Manually (page 23).
• When you are outside the office, the IPsec VPN Client automatically prompts you to
authenticate when you access a service or application that requires a VPN. See
Authenticating Yourself (page 24).
If you get the login prompt each time you open computer, using "Disable VPN" button should prevent this auto connect from happening.