Can anyone explain how I can write a simple program to control the voltage output on a serial port. I also may be willing to use any pc port if another port would be better to use. Any help would be great.

What do mean exactly or what are you trying to do as far as voltage control?
The handshake signals can be set or reset, but the data pins are serial clocked in and out at the baud rate, to and from a parallel register.
Better still, an 8255 based I/O card may prove a better alternative, this is the type that Camsoft and others use for machine I/O.
See the www.keithley.com site for details and they have C++ and VB6 example code.
Al.