pcscd and securityd

Every so often, my PowerBook is taken over by an “administrator process” that uses over 70% of the cpu (in activity monitor). As I click on the process and ask for more info it shows that the process "pcscd" has as parent "securityd" that has as parent "launchd" whose parent is "kernel_task0". What is this and is it necessary? If not, how to stop the process or prevent the process from starting.