Power problems with Hp Slimline 5212y?

I was given an HP Slimline 5212y tower that worked fine. Suddenly, the computer shut down and all I got was the light on the psu at the back of the case. Nothing else was happening. I figured that the PSU died and I wanted to make this computer more upgradeable anyway. So, I transplanted the motherboard into an ATX Antec case and replaced the PSU with a Raidmax 480w PSU. Once all the connections were completed I turned on the computer and all I got was the CPU fan starting and stopping almost immediately and that light on the PSU. :mad: The monitor either doesn't recognise the computer or the motherboard is fried. I'm hoping that if I install a video card that the problem will be solved.

Has anyone had a problem with the Slimline PSU frying the motherboard when it dies? Please help. I can't afford a whole new computer.