PC starts up but no bootscreen.

Hello,
I have an prebuilt HP computer. The HP model should be "HP Pavilion Media Center a1520n Desktop PC" I am not sure since I when we bought this it was for my girlfriend and she has had it at her new apartment, but I took it home with me just the other day due to the failure. I brought it open all secured and in anti-static wrap and only took the mother board, Power supply, and the ON button.

I have tried the eraser to the RAM modules, also did that to my PCI-E video card. I though initially it was a Video card problem. Although after testing the on-board graphics that is on the Motherboard and resetting it with the CMOS battery no luck.

So my first attempt to connect it without the tower laying down on my PC table it booted up, and I had no hard drive in it. Sata hard drive is what comes with it. I connected it to a tower and I had it leaning on my desk so I can connect everything and I kept it leaning and turned it on and the hard started to load XP and everything turned on. I was like that was easy? Then I Turned it off and Put it straight up and it would not load. I tried putting it in the same position and taking it out of the case and still no luck.

I am back at first base. The CPU seems to be working as it heats up after a few seconds. Not to fast though, I could touch it and it doesnt get nearly as hot as my other ones. The fans all work.

One thing that gets to me is that when I remove the Ram I get no Post message or beeps. Shouldn't that mean the motherboard is dead? I ain't an expert with this, so I come here maybe with someone who knows.

Some of the information on the PC.

motherboard: A8N-LA

RAM: 2X Hynix PC3200U (512MB DDR 400mhz CL3) 2X Samsung M368L3313CT1 (256MB DDR PC2100 CL2.5). I installed the Samsung Ram modules later on and never had problems. I am testing the Motherboard WITHOUT them as they aren't part of the stock build.

Video: PCI-E e-GeForce7200 256MB DDR2

CPU: Athlon 64 X2 (T) 3800+ 2.0 GHz

PSU: Bestec ATX-300-127 (Output 300W MAX)

If anyone needs more details or wants me to do something just let me know.