Our customers often have many different types of equipment at their facilities. Many want to administer that equipment themselves, since they all have PLCs and CNCs that need programming, tweaking, or backing up.
They buy a laptop and the software, and realize that only SOME of it is working correctly. Very often it is because another program is latched onto the serial port. We see this constantly with Rockwell Software's RS Linx.
By default, RS Linx sets itself up as a service under windows, and is constantly monitoring the serial port for connection to Allen Bradley PLCs and equipment.
Then if you try to run Siemens' Step 7 or Step 5 (or virtually any other serial based comm program), it can't communicate because RSLinx has control of the serial port. The quick fix is to right click on RS Linx down by the clock on the taskbar and close it. it will release the comm port, but when you reboot, it will come back.
A perminant fix to disable the RSLinx service, so that it only runs when you tell it to.
In your start menu, locate the Rockwell folder, look in the RS Linx folder for a program called "RSLinx Classic Launch Control Panel".
You can stop (and start) the service there, and if you uncheck the "always run as service" box, it won't start back on reboot.