1.) PC wont boot into BIOS stage, CPU light on, fans running, monitor light blinking green/yellow (not orange) - RAM problem? or something else?

2.) Is a DDR2-400 RAM module compatible with a board that supports DDR2-667 and DDR2-800?

I have a friend who unknowingly fried his motherboard. i don't know how exactly, all i know is that he was swapping out CD drives and poof! wont boot. we troubleshot and found out that it was the board. the CPU was an HP Pavillion a1228d unmodded. so we bought a new motherboard.

we assembled the unit and found the stated problems. we figured out these:
-if hard drive was the problem, it should boot to bios and say hard drive not found or something. but its not booting to bios so it should not be the harddrive.
-if it were the board again, it would shut the whole thing off and not even move the fans.
-same goes for the processor and PSU, if they were culprits, it should not even turn the fans as well as light up the CPU.
-no other connected hardware except the monitor, mouse and keyboard.

so we suspect the RAM. the RAM the old board came with was a 256mb DDR2-400. In the new board manual, it says it supports DDR2-667 and DDR2-800. i read wikipedia and says that RAM is backwards compatible but dont understand in which manner exactly. isnt it that newer boards support older modules at lower speed, or is it the other way around, where the older boards run the newer modules at lower speed?

Try taking out the Ram, Mouse Keyboard and Graphics card.

Boot with only the Processor and heatsink/fan on.

You should hear a succession of beeps 3 or4 evenly paced.

If you don't hear this than there may be an issue with the processor or motherboard.

In your case it may more likely be the processor but don't rule out the motherboard, it is rare but you can get a new faulty motherboard.

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