Question : Why  wont a hp laptop power up?

I have a hp pavillion 9000 laptop that wont power up.  I went and got a different power adapter, I have changed memory, took out the battery and then used the new power adapter and even was able to take out the motherbd battery but there is now power.  The only light that comes on is the power button light when you push it and then it goes off when you let go.  It soulnds like a motherbd but not sure

Answer : Why  wont a hp laptop power up?

It seems that you may have narrowed it down to the motherboard.
You have covered many of the troubleshooting steps to rule out possibilities.
1) You've ruled out the adapter.
2) You ruled out faulty socket power as the laptop power light comes on. (Believe it or not I've seen systems that were "broken" due to a faulty wall outlet that simply didn't function).
3) With faulty memory or CMOS battery you would normally still receive some sort of POST code (beeps) at power up.
If it was my system I would now look at the motherboard.
One other test you could run is to see if the laptop will power up with only the battery and no AC power. Chances are it won't. If it does though, you've identified a faulty AC circuit/connection on the motherboard.