I had almost the same problem and I saw a article on here where someone re-installed VMware and it cured their problem. I am using VMware workstation 8.04 trying to keep a old NT 4 machine clunking along. We are using it to download our Siemens operator panels via serial. I installed the Vmware software then plugged in a Prolific USB-Serial adapter. When I tried to connect the serial port it gave me the error: Unable to attach serial port, path not found. It did this with pretty much any virtual machine I tried.
From what I gather if you install Vmware on a PC with no serial ports available, the virtual driver for the serial ports doesn't install correctly. (I'm guessing here) So I plugged in the USB-serial, verified it worked correctly on the host and then just did a 'repair' on VMware. Worked like a charm.
Now I just wish I could 'map' COM3 to COM1 on the virtual. The old software on the NT machine only supports Com1-Com2.I can do this with MS virtual PC but that doesn't appear to be a option with VMware.
Either way Vmware has really been a blessing here! We were running out of PC's that could run NT4!!