Google (search terms: "Serial port designations in Win32") is your friend (sometimes):

http://www.tetraedre.com/advanced/serial2.php?new_lang=en re Development of VHDL & Verilog modules

which says, among many other things:

COM10 and above
On Microsoft Windows, you can use COM1 to COM9 to access serial ports but to access ports with numbers above 9, you need to use the following port name : \\.\COM10.
Note that it is also possible to used \\.\COM1 to access COM1.

Don't know if this is relevant to your problem but here's hoping it some help.