I had zero luck directly binding a USB-to-serial adapter to a XP VM. It would bind, but transfer speed was very low and only worked for small data bursts. The fix was to set up a virtual serial port using a server socket. The serial client (Mac side) is provided by a small application which you can find at: 'http://eudyptes.com/SerialClient.php' . So far it has worked perfectly with everything I've tried. Many thanks to the folks at 'Eudyptes Info' for the fix.
Probably depends on which chip the USB-Serial convertor uses. I tried that program with a FTDI based one and "SerialClient" died. But it turns out my FTDI based convertor worked OK using USB support based in Parallels along with the Windows2000 driver from FTDI.
