Which USB to serial converter, what speed do you have the converter set to and what operating system?
I've just spent most of the weekend getting an old Aten converter running on the 64 bit version of Windows 7. Even once I had the right driver running, the port and the program had to be running at the same speed - which doesn't happen with real serial ports. (That "speed" in Windows is really a sort of speed limit - the port will run at any speed up to that one and, with some RS-232 hardware, at all speeds. Not so with USB adapters - in Windows 7.)
Play around and you may hit the right combination.
(BTW, for anyone trying to get a Belkin 409 adapter running in 64 bits, don't waste your time. Great adapter - if you have a 32 bit OS.)