Command for opening serial port in Windows 7

Maybe you can use the Powershell? It's included in Win7...

code taken from here http://blogs.msdn.com/b/powershell/archive/2006/08/31/writing-and-reading-info-from-serial-ports.aspx

Writing to a Serial Port

PS> [System.IO.Ports.SerialPort]::getportnames()
COM3
PS> $port= new-Object System.IO.Ports.SerialPort COM3,9600,None,8,one
PS> $port.open()
PS> $port.WriteLine("Hello world")
PS> $port.Close()

Reading from a Serial Port

PS> $port= new-Object System.IO.Ports.SerialPort COM3,9600,None,8,one
PS> $port.Open()
PS> $port.ReadLine()

To send contents of a file to a serial port:

copy file.bin com1 /b

To send a few characters to a serial port:

echo hello > com1