Controling an XD-40 LCUS USB relay
The problem is that it isn't expecting an ASCII string like "A00100A1". Instead, it's expecting 4 raw bytes: 0xA0, 0x01, 0x00, 0xA1 (shown as hexidecimal).

You basically need to write a program to open the serial port, set it into "raw" mode so the OS doesn't try to change any bytes, and sent the four binary bytes. I could help you with Linux C code but it sounds like you're using Windows and I don't have the knowledge there.

Edit: See this Superuser page for suggestions for sending raw bytes to a serial device.