Software :: Internal HD Won't Load Mac OS X Leopard?

I have had Tiger for a few years now, So i finally decided to upgrade and bought Leopard 10.5 from an online website, I installed it on my mac and when i try to boot it up on my internal harddrive (where i installed it) all it does is just boot up.. go to a blue screen for 2-3 seconds and then it alternates between a black and brown screen with a beachball as the cursor.. So i've tried to reinstall and all that stuff and i even partitioned and installed it on my External 1T harddrive that i just bought and i am now booted up on that.. When i installed it on my 1T all that happened during bootup was a short delay at the blue screen 2-3 seconds.. and then the brown screen came up followed by a black screen which lead me to the welcome video/intro type thing. I've tried running the machine on my internal harddrive for several minutes but that did not provide any results either. I've tried to repair permissions, repair disk, erase and install all that stuff. The HD "appears to be okay" so i don't know what to do anymore... I have also tried to delete certain files such as the application enhancer files and that has also failed.