Vista - Fast User Switching via Command Line or Batch File?

Is it possible to switch users (via fast user switching) by running a script
or executing a program? I would like to automate the process and map it to
a function key on a Logitech keyboard. I'm currently running Vista
Ultimate, have only two user accounts, and both are set up as administrators
with no passwords.

I'd like the process to just switch to the other user - not take me to the
user selection screen (there are only two users). The Logitech keyboard
mapping software can map any number of keys to launch a program or run a
batch file.

Thanks in advance for your help.