I ran the installer (mbedWinSerial_16466.exe) (on Windows XP Pro, Version 2002, Service Pack 3). A new port (mbed Serial Port, COM3) appeared, but with the error "This device cannot start. (Code 10)".

In Device Manager, the port disappears/reappears when I unplug/replug the mbed; but always shows the yellow-circle-and-exclamation-point icon indicating a problem.

Hyperterminal will let me configure a session using this port and even acts like it connects to the port, but no characters ever come in from my test program.

I have tried uninstalling and reinstalling the driver several times, using different cables and on different USB ports. I have tried re-downloading the installer using Chrome, FireFox, and IE, including direct Run rather than Save (IE). I have made sure there was no Explorer window open looking at mbed. Always the same result.

I researched "Code 10". It means "there was some problem on a deep nether level".

I am out of ideas on how to investigate this further.

I had a similar problem. For me it helped to change the port number. You can change it in the device manger. Under advance properties for the port device.

Some progress:

I changed it to COM27, the first one not already "in use".  It did not show with the new port number until I closed and re-opened Device Manager.  I had to disable and re-enable the port to get the yellow "problem" icon to disappear.  Now my system thinks it is there, but when I try to connect to COM27 using HyperTerminal, I get the error "Another program is using the selected Telephony device.  Try again after the other program completes."

Re-booting did not help.

Does my system think both mbed and HT are on the same "end" of COM27?

The solution to this was cryptic. Perhaps typical of real puzzlers, it was interference from completely unrelated software.

I noticed my Palm would not synchronize, which it usually does by Bluetooth. I checked which port the PC end was set to use.  It was COM27!

I remember this now. If HotSync Manager ("HSM", the software that the Palm system uses to synchronize between a PC and mobile devices) is active, when you create a new COM port, HSM assumes you want to start using that port for any local synchronization.

This is the first time I have been mad at HSM, but it was a real brute in this case. I could manually change HSM back to the Bluetooth COM port it was supposed to use, but every time I would plug in the mbed, HSM would grab that port again. The solution is to exit HSM entirely whenever I am working with the mbed.

I uninstalled COM27, ran mbedWinSerial_16466.exe with HSM off. This time it installed COM3 just fine. The serial test program is running as it is supposed to.

This might also be a problem if you are using other automatic synchronization software. I believe the Microsoft analog is called something like "ActiveSync".