Make sure your FIFO buffers are set either low or turned off. To do this
Right click on "my computer"
click on "properties"
Go to hardware tab
Click on "device manager"
Click the + in front of "Ports (COM & LPT)"
double click on the usb port
click on "port settings" tab
click on "advanced"
Move scroll bar all the way to the left for both Transmit Buffer and Recieve Buffer
If that does not solve the problem it is one of 2 things:
The flow control settings on the machine and pc may not match. Do you know that the software you are using works with the settings you are using on any other computer?
Many USB to Serial devices do not work properly when you try to use them for serial communication to a machine. One device that I do know works well is StarTech.com model ICUSB232.