What do you call a "invalid output"??? Random characters in the display?

Modify your program to put some data in the display and start using the data from the ADC just after some input from you (some button to be pushed, or a delay of some seconds)

Aren't you using any power-up reset circuit? Look at some small 8051 project and you'll find one to use (a capacitor in series with a resistor)

Something like this:

--------+-----------+---> 5V
I I
I I
o 10uF -----
reset [ ---
button o I
I I
I 1K [ ]
I I
+-----------+--------> pin 9 (reset)
47K [ ]
I
I
GND ---
-

HTH