ram slots
ram slots
You have three functional RAM slots on your motherboard, but when you try the third slot, the system begins to sound an error and fails to boot. Your machine is an older HP Pavilion with an Intel Core vPro 2.4GHz processor, 1GB of RAM (two 512MB sticks), and you're planning to add another 512MB. It's a budget PC, so you're okay with the limitations.
It seems the stick or dim is either broken or dead. I wouldn't invest more in this setup.