can't get Bios up after build ( Asus M Board)

I've spent ages getting 2nd hand bits off e bay and finally come the day carefully mount my Asus P5WD2 M B in case but notice some of the screws don't go in quite right later discover I am using bigger threaded ones and can't get them out now , any way onward I then carefully fit power leads and other smaller leads from on off switches etc ti urn power on and the fans and lights come on wow! But Optical DVD won't open remedy this I take out ide cable and then they open, also notice the Computer won't turn off except by using the switch on the back of the power supply, then add video card and attach monitor zilch no BIOS , trouble shooting I guess it could be the video card ( all 2nd hand components ) ar, also checked the RAM right type have takenn all but 3 sticks out and then just left one ( could it be the RAM), could it be the MBoard ( this was my first buy nearly new and my best purchase !!! OH God have I killed the Mother board , still can't get two of the screws out ( the others are right , think I used case screws by mistake but the circumference of the head seems the same). HELP PLEASE>>>