When the traditional 9-pin serial port was phased out of most PC specs, its function was replaced by USB ports. Adapters are common, and RacerMate provided USB-serial adapters containing devices and drivers by Prolific. They looked like the picture in figure 2, and connected the USB port to the stereo adapter in figure 1.
The short story is this: Prolific has been terrible at providing working drivers for modern operating systems. In fact, they never actually worked completely reliably on Windows XP. The advent of Windows Vista and 7 exposed the problems even more acutely. Prolific's behavior in providing drivers to its customers and cable manufacturers has been a failure. If your PC is failing to blue-screen it is often the drivers for the Prolific devices. This issue affected cycling device manufacturers SRM, Saris-CycleOps, and RacerMate. They have all since moved to FTDI devices, and they all ship FTDI technology with current product. RacerMate has since engaged FTDI to build their own adapter from USB direct to the CompuTrainer stereo cable. It looks like Figure 3, but may also appear as a one piece unit with no wire.
Rather than confuse you showing how to install imperfect workarounds for Prolific drivers (and leave you dealing with machines that will still BSOD on you), our recommendation is just this simple: Ditch your Prolific-based adapters, and get USB-Serial adapters that use FTDI technology. Period.
There are many others.
We do not provide support to users who insist on using Prolific devices. They are flawed and are the first source of any trouble you may be having with PC to CompuTrainer or VeloTron communications.