If you have your GSM mobile phone connected with the PC, you can send SMS to any mobile from your PC.
Many mobile vendors like Samsung, Nokia etc. provide application for this purpose in the connectivity suite itself (which comes free with the mobile or can be downloaded from the vendor's website).
However, there are other mobile vendors which don't provide an software for this out of the box, like Sony Ericsson. But, you need not worry if your connectivity kit doesn't contain a software for sending SMS, because for this we have a good and free addon for Windows XP in the market from Microsoft which is called as "Microsoft SMS Sender".
Microsoft SMS Sender supports only SMS (and not MMS etc) over GSM networks (no CDMA/TDMA support). Though it has many limitations like this, it does the basic job of sending SMS nicely and the best part is that it can be used as a command line tool. It supports various command line switches and hence can be automated using the command line options. For details on command line options, refer to the documentation in the installation package.
