I have recently aquired a dell dimension 4300 from a friend of mine that was partially disassembled. everything was there so i put it back together and now when i go to power it up all i get is three beeps and the monitor shuts down. I dont even get a picture on the monitor before it powers down. on the back of the tower where there are the four lights (a,b,c,d) a & b are yellow and c & d are green. anybody have any ideas before i go beeping crazy?