i am afraid this is not an easy task. Once i had programmed similar problems using plain c under real mode, what is rather outdated. Today i am using professional library for port and memory mapped IO on XP. Maybe you will succeed by using this interesting library: http://www.codeproject.com/KB/system/serial.aspx
My electrical is rusty but i think t=RC will work. But I suggest you to do lots of research before proceeding with it. Pins from the serial port have physical voltages of their own. This is a fairly difficult problem and you may end up frying your port. You are better off with a multimeter.
Serial port behaves erraticaly and produces +3 to +25 V for logic 1 and -3 to -25 for logic 0. I suggest you use a Max232 converter IC to scale them down to static +5V and 0V.