232 to 485 circuit
there are two projects on net which may be usefull for you,
1st google for "Igor USB", which someone from slovaki made a usb to serial converter using ATMEL AVR microcontroller.
2nd in sourceforge.net look for com0com project which emulates a com port in system.
idea: you can implement your own usb to rs232 converter using igor's circuit, but it did not add a new com device to system. use com0com to define one, and redirect transactions to your own hardware (Igore).
good luck;