The host PC has a high speed serial card fitted ("Brainboxes" RS422/485 PCi), plus its respective driver software. In Control Panel -> Device Manager the card's two serial ports appear as "Brainboxes RS422/485 Serial Port (COM8)" and "Brainboxes RS422/485 Serial Port (COM9)". They work fine when I talk to them from the host PC.
I now need to talk to the com ports from the guest machine. I've configured the VM's settings to expose these serial ports, and on the guest machine (Win7) I can see them in Control Panel -> Device Manager, appearing as COM2 and COM3. Unfortunately my software is unable to talk to them, failing to open the serial port. The error message is "The parameter is incorrect", which I believe is due to using a high baud rate (921600) which isn't supported by "vanilla" com ports - hence the need for the Brainboxes driver.
I installed their driver on the guest machine, then tried to update the two com ports (COM2 & COM3) with this driver. However the guest PC crashes at this point with a blue screen of death. Occasionally it will update the driver, but when I restart the PC I get the blue screen again and have to run a Windows repair to rollback the driver installation. Any thoughts?