Computer authentication by IPSec is performed by using preshared keys or computer certificates. A pre-shared key identifies one party during Authentication Phase. Per definition, "Pre-shared" means you have to share it with another party before you can establish a secure VPN tunnel.
The strongest method of authentication is the use of a PKI and certificates. However, smaller organizations cannot afford the implementation of a PKI system and a well managed preshared key method can be easier and just as powerful.
TheGreenBow VPN Client supports both modes.