Dell latitude d600

I recently installed a hard drive in my laptop and after a few minutes of using it it would lock up and sometimes it would give me a grey screen with www.dell.com and tehn turn off. after i would restart lights come on for about 5 sec and turn off, no monitor, hard drive or fan. I noticed if I aplly light pressure in the area where the processor is it boots up, but it will lock up again. I thought the processor was overheating so I applied some thermal compund to it and I am still having the same problem. please help. thank you