Alienware 14, black screen, 8 beeps

Good day.

Decided to send this laptop to my cousin, trying to downgrade to windows 7 from windows 8.1 so he could still play some old games, entered BIOS, select LEGACY boot instead of UEFI, now the laptop won't boot up and I could hear 8 beeps, and it repeats

Laptop condition:

1. LCD display has been removed, along with the all the cables, been running fine for 8 months with 21 inches monitor via HDMI cable.
2. Two months ago laptop starts killing power supply, destroyed a total of 3, power supply will only become overheated and explode while playing graphic intensive games.
3. Currently using a 19v 4a power supply / adaptor (the original adaptor is 19.5v 7.7a), it won't charge the battery but laptop been running fine for 2 months, watching movies and surf the net etc. 

I think that if I didn't choose LEGACY boot this wouldn't have happened, now I just couldn't find a way to enter the BIOS or windows, alienware recovery disk doesn't seems to do anything too. Also, holding fn+PAUSE seems to stop the beeping but I am clueless of what to do next, is there any other key combination or what I should try. Thanks