Not to doubt your experience or thoughts, as I have never tried a PIC and RSLinx with an USB-serial converter. I have used such for hooking up to an Ultra series servo controller, to a Microligix 1000 with RSLinx and a couple other serial devices not related to controls/PLC. I have found that some of these USB-serial converters can be very fussy with the drivers they come with or the device itself, but I have never run into a situation wher eI could not use it. One such USB-serial converter I had to set up on a vendors laptop that came in here requires the laptop to be rebooted fresh each time the data recording module was unhooked and rehooked up to it. That was a cheap Radio Shack converter (Only one available locally that day.) THe drivers also crashed the laptop.
Again, I can not say that I have tried a PIC. Fortunately, both laptops I have at work have real serial ports.