Alright, I suppose I'll bring out my Dashboard viewer yet again. It's written in C/C++ and runs under Windows. It does read data in from the serial port. In short, the way to access a COM Port (aka serial port) is to simply use ReadFile/WriteFile on it. There's a bit more too it but you can find all of it in the Dashboard Viewer.
I haven't played with it too much in several years as I haven't had too much use for it. It does include the full source code. It can be found here:
http://www.bcpl.net/~mleese/dashboard.html
I hope you find it useful. Feel free to contact me if you have any questions.
Matt