Thank you all very much for your help so far. I was able to get access to a PXI system running WinXP. After connecting the instrument to the on-board serial port of the PXI system and running the test vi at a loop rate for 10ms for over an hour, the system did not crash. I am now pretty sure that the driver for the serial adapter that I am using is not fully compatible with Win7-64bit even though they claim it is. I also tried out other USB-serial adapters and one seems to be working fine so far. I think, we can close this thread.
Thank you again very much for your help.
PS: In this application, I am looking for a terminal character to appear and set the byte count to a large number. The expected responses are never larger than about a dozen bytes.
