Here is how I installed hl340 cable driver in windows7 Professional

Method1:
1. At first I removed the protection from system drive (i.e. partition in which your operating system is residing). In my case it was C:
Steps : System properties -> Advanced System Settings -> System Protection
Select System drive then click on configure, then choose option which says, turn off system protection.

2. Then I restarted the computer and hit f8 continuously to bring the startup menus (like one to start safemode in XP), from the options I choose to disable the driver signature.

3. After computer comes to working state, I installed the driver (Download Link), which I had downloaded from the cable manufacturer’s site. (For details view my previous post on vista)

4. I set the setup file in compatibility mode (For Windows XP SP2003). Then I ran the setup, which I already had extracted.

5. I got Drive Install Failure dialog box. Then I plugged the cable, windows starts to search for the drivers, I choose to manually install the driver, then I located the driver manually, and when asked choose to continue anyway with driver installation.

6. Then I tested it with HyperTerminal and also with Java Communication API, and it is working great.

Remember, You will have to disable driver signature verification every time you restart your computer, if you are willing to use USB to serial communication otherwise the drivers for HL340 cable can’t be installed, when you plug it to computer.

And the other problem I have seen so far is that, each time it is installed to computer, the PORT seems to be used and not released and thus system assigns the new address to the port i.e. each time you plug it to computer, a new address is assigned, suppose first time you plugged it, it assigns say, COM3, then when you use the cable second time, it assigns say COM4, and so on. And the total ports that can be used is upto COM255.

Let me know if you are having any problem during installation, please post error you received.

Method2:
If that doesn’t work you can try this one:
Stop the Hardware Policy Driver from. At first it is hidden, you have to enable the show hidden devices by clicking on the view > show hidden devices in device manager menu.

Then under non-plug and play devices, you will find the hardware policy driver, double-click on it, then select driver and click on stop. Under startup select demand.

Now search for another driver named Windows Firewall Authorization Driver under non-plug and play and repeat the same steps as you did for Hardware Policy Driver.

Then restart the computer by disable driver signature.

Once computer is in working state, Select the driver setup file, right-click on it, select troubleshoot compatibility, then select try recommended settings, and then click on start program. When asked to install driver select continue anyway.

This helps sometimes, I got it work twice, my luck.