I'm not sure what you mean by turn the 38kHz on or off with the serial in. I suspect you mean how to modulate it with serial out?

Since you haven't said what PIC you're using I'm going to assume it has a CCP module. Set that up to provide the 38kHz carrier, connect the IR-LED between the PWM output and the serial output (with a current limit resistor). Depending on which way you connect the LED you'll get "true" or "inverted" output.

If you want to push more current thru the LED than the PIC can supply (~25mA) then simply AND the two signals (PWM and serial) and use the output to drive the LED via a MOSFET or whatever.

/Henrik.