I have two Nano here. It was succesfully updated, but cannot flash the compiler. It says, Serial port 'COM25' already in use. Tried with 'Arduino Nano w/ ATmega168' and 'Arduino Nano w/Atmega328'. Same error. The tutorial has not solved my problem. I'll throw Nano in the trash. I don't know what to do with that.
Later edit:As it turns out the problem is caused by counterfeit chips, I must delete the workaround procedure I have found, because altering device drivers is contrary to the FTDI license agreement.If you have such a fake chip, please contact the seller for reimbursement.
I have just had this problem too.It was working fine on windows 8.1 for the last month, then all of a sudden. Missing Drivers problem.I got the drivers from FTDI and re-installed using the .inf files and the .EXE installer. No difference.I was left with a missing UART Driver.(This Works for Me, try it at own risk.)To fix it I got the drivers in normal format with the .INF files. Extracted the zip.From ControlPanel->Device ManagerLocate the missing UART driver. Right Mouse, Update Driver, Browse Computer -> Let Me Pick->Have Disk->Select the Location of the drivers (port.inf ) and OK.Now you have a list of drivers to pick from. Choose the USB Serial Converter. OKAfter that Another Missing Device Will appear, or Should, if it doesnt then this isnt going to work and uninstall the driver you just picked.Repeat the above steps and install the USB Serial driverThis should now work. If it doesn't go an uninstall those drivers by right clicking the device and uninstall in driver manager.MS must have done something to cause this in an update as this was fine a few weeks ago, hope this helps some of you having the same problem.