Cannot Access USB Serial Converter

I have a USB<->serial converter (several, actually) that I cannot get to work under Windows 7. I know the hardware works.

When I try to open the virtual serial port in a terminal program, I get an error message stating that it cannot open the port. When I try to use the old-fashioned command line method ("copy test.txt com7" from the "DOS prompt"), I get an "Access denied" error message.

More details: My computer is a dual-boot Windows 7 Ultimate 64-bit / Windows XP Pro 32-bit system. The USB serial converter worked immediately under WinXP when I rebooted, and it even works in the WinXP virtual machine under Windows 7. I have the latest certified drivers (from FTDI), which are stated to function under Windows 7 64-bit.

The serial converter appears as it should in the Device manager. My computer has one physical COM port, and I can access that one without any problems.

The serial converter is a USB-RS485-WE-1800-BT from FTDI. I have tried a USB<->RS232 converter (don't have the model number here), but it behaved exactly like the other one. I also started the terminal program as an adminstrator (since I suspected access rights issues), but it did not help.

I have tried uninstalling the drivers, then installing the latest certified driver and the even later non-certified driver, but still had the same behaviour.