Try this and see what happens, should give you a 50% duty cycle ~36khz square wave on your o'scope:
Code:
@ DEVICE pic16F877, HS_OSC ; System Clock Options
@ DEVICE pic16F877, WDT_OFF ; Watchdog Timer
@ DEVICE pic16F877, PWRT_ON ; Power-On Timer
@ DEVICE pic16F877, BOD_OFF ; Brown-Out Detect
@ DEVICE pic16F877, CPD_OFF ; Data Memory Code Protect
@ DEVICE pic16F877, PROTECT_OFF
DEFINE OSC 20
load con 65468:intcon=$c0:option_reg=0:adcon1=$87:t1con=1:pie1=1:c var byte
goto main
ON INTERRUPT GOTO SINUS
TMR1L=LOAD.lowbyte:TMR1H=LOAD.highbyte
MAIN:      GOTO MAIN    
DISABLE
SINUS: if pir1.0=1 then
          pir1.0=0:tmr1l=load.lowbyte:tmr1h=load.highbyte:c=c+1:portb.0=c.0
          endif
RESUME
ENABLE
END
And take note of Mr_E's note about adding values.