Need Help with Motherboard and GPU

I'm trying to install a Radeon 5570 in an old HP computer.I used to run a 9400gt in it but then i got a new computer.Gave it to my brother and now he wants my old GPU in it.I've replaced the PSU to a Apevia 500watt(which is way more than it needs) but i can't get it to boot with the 5570.I've tryed disabling the on board graphics thru windows but that didn't work either.I would try in the BIOS but since it's an HP the BIOS are locked and their aren't many options.I've tryed downloading the drivers from ATI for 5570 but no luck.

Core 2 Duo E4400 @2ghz
Asus P5LP-LE motherboard
3G DDR2 RAM
Apevia 500watt PSU
Windows Vista 32bit

I used to run a 9400GT in this computer but i can't get the new 5570 to work in it.Their has been a fresh install of Windows so none of the old 9400G drivers are on it.I know it's not the 5570 thats the problem because i was using it in my computer before.

Any help would be apreciated,and if i'm in the wrong sections asking this question will a moderator plz move it.