Windows NT does not allow the use of COM3 or COM4 by default. To override use the following registry hack, add a string value called COM3 if you want to enable COM3, or COM4 if you want to add COM4 support. The text string is the same as the value name (com3=com3). To enable, apply the following Windows NT Registry hack :

Hive: HKEY_LOCAL_MACHINE
Key: hardware\devicemap\serialcom
Name: COM3
Name: COM4
Type: REG_SZ
Value: COM3
Value: COM4

You can have a problem is getting com1 and com3 to work together or com2 and com4 to work together. com1 and com3 share an interrupt as does com2 and com4. You can assign two COM ports with the same interrupt as long as you don't use the ports at the same time. For example, com1 to serial port used by a digital camera and com3 to a serial port used by a scanner. As long as the applications are run separately and do not attempt to access the ports simulataneously, they will both work. Try to download images from your camera and use your scanner at the same time, NO. Then you run into a resource clash.