http://www.totalcardiagnostics.com/support/Knowledgebase/Article/View/92/20/prolific-usb-to-serial-fix-official-solution-to-code-10-error

Download and save the "PL2303_64bit_ Installer.exe" by using "http://www.totalcardiagnostics.com/files/PL2303_64bit_Installer.exe".
Unplug all USB-To-Serial adapters from your computer - and double-click on "PL2303_64bit_Installer.exe"
When it prompts you, plug in one (1) of your USB-To-Serial adapters and click "Continue".
Reboot your computer. That's it!

The "Driver Version" must say "3.3.2.102" dated 09/24/08.

The other solution is to reassign the port number. You can do this via Device Manager.
Go into the Ports (COM & LPT) section, right click on the port you want to change, select Properties, 
go into the Port Settings tab, and click on the Advanced button. 
In the bottom of the Advanced Settings dialog you can change the COM port number. 
The list helpfully shows what numbers have been assigned to other devices, 
including devices which are not currently plugged in. 
If this cannot be done due to the code 31 error, either change the conflicting device's port, 
or unplug the conflicting device, plug in the new device and then change the new device's port. 