16F877A and LCD HD44780

[Archangel]

Try add flags = 0
I usually eliminate POT from circuit & tie pin 3 to ground
The PIC 16F877a has power and ground pins on both sides of the chip, make sure both sides are hooked up
I did not see any config statement in that code, so how it is running or not is anyone's guess, are you sure it is? Try making a blinking led to be sure.

The code below is the 16F877A.inc file from my v2.6 install

;****************************************************************
;*  16F877A.INC                                                 *
;*                                                              *
;*  By        : Leonard Zerman, Jeff Schmoyer                   *
;*  Notice    : Copyright (c) 2003 microEngineering Labs, Inc.  *
;*              All Rights Reserved                             *
;*  Date      : 11/06/03                                        *
;*  Version   : 2.45                                            *
;*  Notes     :                                                 *
;****************************************************************
        NOLIST
    ifdef PM_USED
        LIST
        include 'M16F87xA.INC'  ; PM header
        device  pic16F877A, xt_osc, wdt_on, lvp_off, protect_off
        XALL
        NOLIST
    else
        LIST
        LIST p = 16F877A, r = dec, w = -302
        INCLUDE "P16F877A.INC"  ; MPASM  Header
        __config _XT_OSC & _WDT_ON & _LVP_OFF & _CP_OFF
        NOLIST
    endif
        LIST

Notice it says XT_OSC, this is incorrect for your 20 mhz osc, it should say HS_OSC, for any resonator and crystals above 4 mhz, this is an oscillator power setting, either change it here or comment this line out and put the config in your code. This statement assumes you are using a version older than 3.0

It is always better to copy paste your code than a link to somewhere,however; a link does make a good supplemental.