"I am using VBasic MSComm Control in windows98 or XP."

In that case, you need to read-up the VB documentation on the MSComm Control!
Sorry, I don't do VB.

We're getting off-topic now - you'd be better to find a PC/Windoze forum to discuss the details of Windoze programming.

The MSDN is an exellent source of Windows programming info; eg
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wmeother/hh/wmeother/comm_277a.asp

You should also visit Jan Axelson's Serial Port Central page - it is required reading for anyone doing anything with serial links:
http://www.lvr.com/serport.htm

and see also ePanorama:
http://www.epanorama.net/links/pc/index.html
http://www.epanorama.net/links/pc/programming.html