Hi!
Well the problem is obvious, you are not switching the banks.The registers in PIC Microcontrollers are banked try to compile this
LIST p=16f876, R=DEC
#include "p16f876.inc"
org 0x00
bsf STATUS,RP0
MOVLW H'06' ;porta all digital io
MOVWF ADCON1
movlw 0x00
movwf TRISB
movwf TRISA
movwf TRISC
bcf STATUS,RP0
movlw 0x0F
movwf PORTA
movwf PORTB
movwf PORTC
circle goto circle
end
in addition to this also be careful about the fuse settings.
one more point is that porta is also the analog input port therefore it is necessary to configure it for digital input/output by writing to the ACON1 register.
Best of Luck
Regards.