to send sms from pc the hardcore way you'll need a cell phone attached to the pc via a serial cable. some can do it with usb but most have problem since the drivers tend to run low level code which makes usb to serial adaptor useless. this is rather difficult to do since you also need to encode the message and stuff.
the easy way is to go with some sms message gateway providers. they once you subscribe to them, all you got to do is call one of their method, pass in appropriate parameters and send it. in most cases they also have email to sms which allows you to send an email to their designated address and your message will be processed and sent out as an sms message.
quick and easy way to do this is like dgorka has said, send a short email to the 10 digit phone number.