I have worked with GSM/GPRS Modem (Siemens TC65 and SonyEricsson GM29). Communication with modem was done serial (com port). Application was developed in C# and it was quering the SIM card for saved SMS's, it could send new SMS messages, and save SMS messages in files on the computer.
To be able to send SMS's from your computer using the phone, it should be connected to the computer and have the communication driver installed. It will create a virtual serial COM port on your computer and my application will send AT commands (SMS message) through these virtual com port. It will query the phone to check if it's connected to network provider and so on...
It can be a simple DLL, that you can integrate in your application and make use of public functions like sendSMS(string phoneNo, string message, string key), etc..and/or an ActiveX component that you will register in your Windows System.