UPDATE: FTDI has bowed against the backlash and this is no longer an issue, see post #11 for details.
I'm not sure how many of you have heard this, or even have understood this, but there's some dodgyness going about the electronics world currently by FTDI.
FTDI is probably the most popular manufacturer of USB > Serial converters on the market. If you have any device out there which identifies to the computer as a serial port (i.e. most ASCOM compatible devices) then there's a very good chance it has an FTDI chip in it. Shoestring cables typically have FTDI based serial converters in them, anyone who's messing with Arduinos will come across them, and there's a good chance that many other devices do too, because coding for a serial port is easier than coding for USB, and it means you don't need to licence a vendor ID.
Anyway to the crux of the issue:
FTDI is one of the most counterfeited brands in the electronics world.
A few days ago FTDI published via Windows Update a new driver. In the past there have been driver updates which prevent the counterfeit devices from working properly, randomly dropping out etc.
The new driver intentionally bricks the counterfeit chips.
The problem is there's really very little way to tell if a chip is counterfeit or not without looking at it under a microscope. There are legitimate vendors who are happily using counterfeit chips, there are those who are unknowingly using counterfeit chips, and there are users who don't realise that buying a $10 shoestring cable is too good to be true when the FTDI chips cost $20 ea in sets of 100.
The driver detects counterfeit devices and sets the USB PID (Product ID) to 0. Windows doesn't like this and the device will drop out, and won't work again on any computer you try.
One solution appears to be to manually force the installation of an earlier driver. So far the forum with the solution is in Russian, but if anyone comes across the problem post in here and I'll talk you through the solution.
Right now the driver affected is version 2.12. It is being rolled out via Windows Update. However
http://www.ftdichip.com/Drivers/VCP.htm FTDI still publishes older versions including version 2.08 which is recommended to fix the problem.
So yes post in here if anyone has problems with COM ports suddenly no longer working this month.