Also, for the Voltage conversion ...

The way you are doing it will overflow the variables and isn't very accurate.

A better way would be to use DIV32 ...
Code:
CHECK :
  ADCIN 3, ADVAL
  VT = ADVAL * 5000  ; 5V with 3 decimals
  VT = DIV32 1023    ; / maximum A/D reading

  IF VT > 550 THEN
    GPIO.5 = 0
    GPIO.0 = 0
  ENDIF
RETURN
hth,