Tom,
You should be able to use the .NET CE Framework (from within Windows) to accomplish this.
Sending SMSs from your Microsoft .NET Compact Framework-based Applications
.NET Compact Framework has built-in handling for sending SMS-s and PDU over a GSM operator and it's fairly well documented.
Also, while not straight .NET, you have options available in the base mobile/broadband api's, it's straight Windows, and can be wrapped/imported using standard .NET technologies.
http://msdn.microsoft.com/en-us/library/windows/desktop/dd323268%28v=VS.85%29.aspx
