Windows 7: Dell computer stays in POST

Hi, I have a Dell Vostro 200 computer.
OS-Windows XP. 2*1GB DDR2 Ram memory, NVIDIA 9600.
I'm not sure what my other components are.

Well a time ago I had this exact same problem. En I had 2*1GB DDR2 RAM memory and 2GB 800 DDR2 RAM memory. After unplugging each component to see which one had the problem, I came up with the 2GB RAM memory.
I then had Windows 7.
My computer was doing fine.

I then installed Windows XP, with 2*1GB DDR2 ram.
I installed Sandra Lite 2012.
When I tried to run it I got a blue screen with white letters saying ( I only read one sentence then it was gone) "There is an error......- Restart"
My pc restarted.
I tried to run it 2days later- same thing happened.
After 1 day I was browsing and got that same message out of nowhere and out was my pc.
If I turned it on, it was in POST, pretty long, my MOnitor shows "DELL" then the ussual "auto detect" and then it goes to sleepmode.

Does anyone know this problem? And what I have encountered?


After this I pulled out all components again, put them back.
It did turn on this time. Took out everything of sandra Lite.
Did not reinstall Microsoft.

Turned my pc out one day. The next day it couldn't turn on again.

Can someone tell me what happened here?