I don't know much about VB. I saw people using MSCOMM32.OCX for serial communication.
Is link below useful?
If you are using VB.Net, you should read this article.
http://www.innovatic.dk/knowledg/SerialCOM/SerialCOM.htm
What the article say:
WARNING! This description is based on VS 2005 and .NET 2.0. Unfortunately, SerialPort does not work in all versions of .Net.