Older software and some older devices rely on hardware flow control on the RS-232 port, on older systems these flow controls tied to the IRQ (interupt signal) input to the CPU (processor).
Originally there were 15 hardware IRQ inputs, Win XP added "virtual IRQ's" that can go as high as 64 I think. Also USB serial adaptors add an additional "bridge" between the CPU's data bus and the serial UART chip (serial controller).
These additional hardware and software "bridges" aren't 100% backward compatable as they add timing issues on signals and can place the port itself off of it's normal default addresses and IRQ.
First step would be the software vendor for recommendations, otherwise try changing the COM port settings to hardware control as opposed to Xon-Xoff or software control and dropping the FIFO buffer down from it's default high setting.
As far as USB - serial adapters the Belkin has the least compatability issues I've seen, but I've also seen software that absoulutely will not work with them, typically from vendors that refuse to update their version 1.0 program from 10yrs ago.
