Boot up problem with Windows Vista.

I have this problem for 4 months now. My windows Vista wouldn't boot up on the system. It shows different error messages at different times. There are two error messages:
1. Memory error ( 3blinks)
2. BIOS error( 2blinks)
I went to the expert and servicing else. They fixed the problem and commented that was happening due to overheating. They adjust 2 GB into the first slot. Then there was no problem. But the problem has recurred again. I bought two memory chips(2 GB and 1 GB). But this new memory chip cannot fix this problem.
It is HP Pavilion Dm4-1160us.
If anyone help me I will be grateful to you.
