UPDATE:
Kenwood has released updated drivers which should resolve any further issues. Please find the new drivers and an installation guide at this link. If the drivers continue giving errors, please attempt the solution below.
Installation of drivers for the Kenwood TH-D74.
All devices attached to your computer via USB need drivers to tell the computer what the device is and what can be done with it. The TH-D74 is no different. There are not electronics in the cable; however, there are electronics in the radio that allow it to connect to the computer. Those electronics use Windows System drivers and installation is seamless on most systems.
The driver process loads three parts. The problem comes if your system fails to install CDC-ACM . 
This is not unique to the Kenwood radio. Other devices suffer from this same problem since the problem lies in the system files on your machine.
We offer this fix. It is not pretty. Warnings will be raised. But it is safe and it works.
- Download this zip CDC-ACM Driver Repair file paying careful attention to where it is put in the download process (you'll have to find it again later).
- However you do it (I was able to just copy out of the zip file to a location on the hard drive), you want to end up with a "usb_driver_installed" File Folder.
- Once you have this as a File Folder (verify this in Explorer), you are ready for the next step.
- Access Device Manager.
- This is how you will find the radio:
- Right click on the designation and select "Update Driver Software"
- Then "Browse my computer for driver software"... that's why we put that file there in the first few steps.
- Click Browse next to Search for driver software in this location.
- Find and select the "usb_driver_installed" directory
- Highlight that directory then click OK.
- Be sure "Include subfolders" is checked... it usually is by default.
- Click Next.
- A warning will be raised as the drivers begin to install... I told you this would not be pretty.
- Take the "Install this driver software anyway" option if you want this to work.
- A success message appears to let you know the process is complete.
- Checking back in Device Manager, you see the device ready to use .
Access the programmer and use Communications | Get data from radio and Communications | Send data to radio with the cable connection between the radio and the computer.