Red light and loud beep occurs when power button is pressed?

Emachine desktop I recently was trying upgrade on working computer using parts from non-working computer added 2 sticks of 512mb ram and computer wouldnt start up also added hard drive from old emachine that when I pushed power button everything seemed to start up except inside a red light was and loud streaming beeb occurs until I held power button and turned it off. So I was going to put the Hard drive in working computer after I took one stick of 512 out and replaced it with stick of 256 pushed power and now my working computer is beebing and not starting. What did I do? What causes this to happen are both computers ruined? or are they both useable.