Several readers have responded to my column on modems (BW--July 10) to point out that some computers will have trouble communicating at 28.8 kilobits per second. But how do you know? From a DOS prompt, type MSD (for Microsoft Diagnostics). Look in the "com ports" section. If the last line, "UART used," doesn't say 16550, you may have trouble handling high speeds. Next step: Buy an internal modem, which bypasses the built-in ports. Or install a high-speed communications board, available at computer stores for around $40.