Okay, I've rectified the code for use the 10-bit ADC. Now I get more accurate temperature measurement.
I've also modified the temperature formula (the bin->voltage conversion is now "integrated" within), so I don't need anymore the bin->voltage calculation effetued by the PIC.

Code:
define OSC 20
DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 3  
DEFINE ADC_SAMPLEUS 10
DEFINE LCD_DREG PORTC
DEFINE LCD_RSREG PORTC
DEFINE LCD_EREG PORTC
DEFINE LCD_EBIT 5 

tb0 VAR WORD
tb1 VAR WORD
tb2 VAR WORD
tb3 VAR WORD
tb3=100

TRISA=%11111111     
ADCON1=%10000010   

PAUSE 4000

lp:

ADCIN 1,tb0
tb1=tb0-16170
tb1=tb1*tb3                                      
tb2=Div32 634
tb0=tb2/100   
tb1=tb2//100    
           
PAUSE 100
 
Lcdout $fe,$c0,"IGBT t.: ",DEC (tb0 DIG 1),DEC (tb0 DIG 0),".",DEC (tb1 DIG 1),"°C"

goto lp