The value returned by analogRead() is a value referenced to the Vcc of the arduino with a possible 0 - 1023 .
If Vcc = 5V, and temp probe voltage is 1V, (measured with a multimeter), then the value returned is
1V /0.00488=204 (5V /1023=0.004887585532746823069403714565 Volts per count)
When you load down the onboard regulator by turning up the brightness on your display (because you are not experienced enough to know that you should never run ANYTHING off your arduino regulator if you are taking analog measurements; EVERYTHING should be powered off a SEPARATE supply) what happens is that your Vcc is no longer 5V. It drops to maybe 4.7V or less.
so now to recalculate
4.7V/1023=0.0045943304007820136852394916911
and
1V /0.0045943304007820136852394916911 =217.65957446808510638297872340447
and so on.
So what is the moral of the story ? (YOU tell me)
PS- google "DS18B20"