Worst computer nightmare.. I think at least

Hello, Problems galore! I had bought a Compaq Presario SR5250nx for college in 2008. To continue my hobbies in video game making I replaced the motherboard with a Asus M3N78-VM. The min power supply is 300w, I then bought a 450W ATX power supply. My CD drive blew out last night it came with the factory. Now I cant install an OS with the new hardware. Then today it was working perfectly well, until I turned on the back power switch then the whole PC came on while I didn't even touch the front panel power button. So I was like ok?? Now no signal is appearing on my monitor. My LG flat panel monitor keeps going into power safe mode whenever the the entire PC is running. I'm thinking my motherboard is either fried or something isn't right with the power setup.