Here's the guide I used to correct the problem :-
HOW TO FIX THE USB-FTDI PROBLEM
(on arduino nano, usb ttl, ft232, ...)
There must have been a production of faulty ftdi drivers (version 220.127.116.11 or later) on august 2014 that makes some arduino boards and other usb devices not readable from the computer. The problem is random, meaning that it is possible to program the arduino a couple of times and then it comes out the ftdi problem, at that point the board isn't accessible anymore.
Once you come up with this problem, upgrading/downgrading the drivers, the IDE or the bootloader is just a waste of time, as the ftdi driver installed leaves some wrong data. This guide followed step by step will completely fix the driver problem thus making your board working again.
Step 1: check if you have a faulty ftdi chip driver
Not every ftdi chip has faulty drivers, so the first step is to check the chip's driver version. In order to check the version you need a software like usbview or usbdeview (both freeware and don't need any installation). Run the software and look for the "USB serial converter" device which is the one related to the ftdi driver, then read the vendor-id and the Product-id whose values should be 0x0403 for the vendor and 0x6001 for the product. The goal is to modify that 0x6001 value with 0x0000, and to do that we need some other software.
Step 2: modify values
Unplug your arduino nano or usb device. In order to modify the driver values you need CDMUninstaller (no installation needed). Fill the vendor and product fields with the values found with usbview (0403 and 6001) and click 'add', finally click 'remove devices'.
Step 3: download the 18.104.22.168 ftdi driver
Go to the ftdi website and download the 22.214.171.124 virtual driver choosing between 32 or 64 bit version, voiding the 'setup executable' file: just download the folder. After downloading the folder, we have to overwrite some files inside of it. Click FTDI.rar to download the files already modified, and place them inside the 126.96.36.199 folder overwriting the old ones.
Step 4: connect your arduino board
connect your arduino board, and stop windows from looking for new drivers. Go to device manager and look for FT232 USB device, then right click on it, select Properties and click on update drivers. Manually select the place in which scan for the new drivers and choose the 188.8.131.52 driver folder with the modified files. Now windows will prompt you saying that it can't certificate the new drivers. Ignore the alert and proceed with the installation. Once installation has finished, windows should recognize the connected board as 'USB recuperacion'.
Close the window and go to device manager, and look for the COM ports device. The connected device is now showing up like USB recuperation by ElectroHobby.es
Step 5: modify the chip eeprom
This is the last step and you need to restore the initial vendor and product values inside the ftdi chip eeprom. In order to do that you need to download and install Ftprog, click on the magnifing glass icon, select 'USB device descriptor' and again select 'custom pid'. Finally click on 'ftdi default'.
A new window shows up and you just need to check flag the device list and then click on 'program'.