You don't need a GSM Modem on the PC end. You can have your "transmitter' send the SMS messages to an Email address. Then on the PC side you read the Email from your mail server - this is very easy to do and there are plenty of examples on the Internet.

Cellular service providers employ an SMS gateway, which is a service that transmits data between different media(i.e. mobile-Internet). This service will have a unique "phone number" assigned to it. To send an SMS message to an Email address you send it to the SMS gateway's "Phone Number". You place the destination Email address at the beginning of your message followed by a line-feed. This format may vary between service providers, but I've worked with clients all over the world, and have yet to see a provider that doesn't support this.

Using the SMS Gateway you can also send an Email to a "cell phone number" and it will be received as an SMS message. Most service providers assign an Email address based on the phone number. For example, to send to an Cingular customer the address would be like this: "2155551212@CingularMe.com".

You can get the SMS Gateway "phone number" and the Email address from your service provider. Also, there are many sites on the Internet that list these items for most providers.