Someone on another forum said that once you have selected USB in WeatherLink 5.8, it changes something permanently that prevents you from going back to using serial via the virtual COM port. He said Davis told him it was a known problem. He said it changes something in the data logger, but I wonder if that is correct. Possibly it changes something in the driver. If I were you, I would completely uninstall the davis driver, reboot, and then reinstall the driver and try to use the COM connection like you used to do. Once you get it back, my suggestion is still one I made earlier. Try VirtualVP. It's a 30 day free trial. It has solved the USB disconnect problem for many people.
I also don't think it's your computer. Too many people have had the problem on enough different computers that that explanation doesn't work for me.
Steve