I don't understand, how a PIC is involveld with your system, cause you can access a GSM module from the PC directly. This would allow an easier programming in my opinion.
Generally, AT commands for SMS handling are using an industry standard command set. They are explained in detail in the user manual of any GSM module, I have seen. They are also supported by all recent mobile phones with a serial interface, but sometimes with reduced functionality, e.g. PDU mode only.