I had a similar problem...turned out to be the USB to serial adapter. They are not all standards compliant. I was able to type on one computer and see it on the other, but I could never do any kind of file transfer...even pc to pc. The best adapters are the ones with an ftdi chipset. They are fully standards compliant. The one I have works very well, and it was fairly cheap ($20). Could be the cable too...
here's a wiring diagram.
