I've had issues with USB-serial adapters for some time, but only recently came to the conclusion that most if not all of the trouble was with Prolific adapters.
I have quite a few Prolifics that I've acquired over many years, and a couple of OEM devices with built in Prolific chips. Everything was working reasonably well for me until Windoze auto-installed a new service pack for Windoze 7. At that point, all of my Prolific devices stopped working on that computer. I updated the BIOS, the drivers for the chipset, and in particular the USB drivers on the computer, and made certain I had the latest Prolific driver - no joy. I went down to Best Buy and bought a new one - which came with drivers. No joy. Meanwhile, devices built on the FTDI chips worked fine. I ordered two new FTDI adapters, which work great on all my Windoze machines, which are at varying levels of Vista, Win 7, and XP.
I've also had problems in the past with Lenovo/IBM desktops dying as a result of USB hardware problems. I am suspicious that the Prolific devices may have been involved as the failures followed the moving of these devices. Fortunately, these were all inexpensive refurb machines that had outlived their usefulness, anyway, so I wasn't out much. So far, the current Lenovo is alive and well.