http://www.cisco.com/warp/customer/110/l2tp-ipsec.html
shows an example config and a working setup.
Microsoft does not support pre-shared keys in remote access scenarios and so encourages customers to use IPSec and L2TP with certs.
However it is technically possible to have pre-shared keys.
Not many are interested because:
1. Cisco VPN client offers advanced features.
2. It's FREE as long as you have a headend VPN device.