LAPTOP- WHEN YOU TURN IT ON IT WONT OPEN TO THE...

WHEN I TURN IT ON IT QUICKLY FLASHES UP THE USUAL START SCREEN WITH INTEL OR ACER WHATEVER IT IS THEN FLASHES PAST THE WINDOWS XP DISPLAY SCREEN AND THE SWITCHES TO A BLACK AND WHITE SCREEN WITH HEAPS OF WRITING SAYING START COMPUTER NORMALLY PLUS SEVERAL OTHER OPTIONS NONE OF WHICH WORK. THEN AFTER SO MANY UNSUCCESSFUL TRIES IT SWITCHES TO A DIFFERENT SCREEN REQUESTING A PIN NUMBER BUT I HAVE NO IDEA WHAT PIN THEY WANT. I BOUGHT IT OFF A MATE WHO BOUGHT IT NEW ORIGINALLY BUT HAVE SINCE LOST CONTACT WITH HIM. PLEASE HELP ME FIX IT MATE