Can't install windows 7 ?

Hi, and before i speak ( which i already did ), sorry if anything is wrong with the thread, since it's my first time here.

My computer Spec :

Motherboard : Gigabyte GA - 870A - USB3
Graphic card : Sapphire Ati Radeon HD 5850 Extreme
RAM : 2 X 2GB PatriotPSD31G123S
Processor : Amd Phenom X2
500 GB Seagate Harddisk
LG DVD ROM

Okay, like the title, i can't install Windows 7. Here's the process :

- I installed windows 7
- I installed drivers of the Motherboard & graphic card
- I installed some softwares ( games, utilities, etc )
- My computer can't start and screen was Black. ( frozen after the safe mode/start normally screen )
- I try to use system recovery, but i got a Blue Screen
- I decided to install windows 7 again
- After the disc loads, the screen goes black again.

More infos :
-The blue screen says 0x00007c error ( and some other stuffs, but i forgot )
-My PC sometimes can display the motherboard intro screen, sometimes cannot.

I tried to have a replace of the RAM from the retailer, but no use, it's the same, but i am suspicious about the harddisk, and the DVD ROM.
Anyone can give a solution ?

A common problem is the wrong RAM or the wrong BIOS settings for the RAM, sometimes the timing setting defaults are not right for the RAM you have etc. I don't know anything about fixing that but that is the first thing you should check.

Then I would try formatting the HD and doing a brand new install ... again.

This bit interests me:
"I installed windows 7"
Did windows boot up normally after this ?

Was there a reason for this?:
- I installed drivers of the Motherboard & graphic card.

Were they not already installed on first boot?
or, did the computer stop booting after this?:
- I installed some softwares ( games, utilities, etc )

And this, which lead into a more difficult issue, which is as Trainable man suggests
"My PC sometimes can display the motherboard intro screen, sometimes cannot"