Computer wont boot now

Hello,
Well im having probs with my comp now. To start, I got a computer given to me from a friend, so I decided to hook it all up and it would not boot, the monitor turns on for a sec thou goes to a yellow light like it is on standby. This monitor did work previously, so I thought maybe it is the hard drive, so i took my hard drive out of the comp that I have that works, and hooked it up to the one goven to me. Well it did the same thing, so I gave up and hooked everything back up too the one that I was using before hand and now its doing the same thing, I also tried switching the memory, well I put all the memory back but maybe it the wrong places, would that cause my probs now, and if so is there a certain way they go back? If someone could please help me that would be great, thanks for your time and I hope to hear from someone soon.
Tranel