OS X won't boot after new install

OK so i installed the leapord on my girlfriends macbook using a external hard drive, everything went fine but after the installation, (which it should restart and come up with the apple welcome screen) it stops at the apple logo, i tried doing the installation again and its the same, any ideas guys?

Thanks,
Yizi

No its a working one, and its been riped from my retail DVD, also used the DVD to check and did the same, so what i did i used the disk utility inside the DVD to erase and re partition the HD to install everything, again everything went fine but now beside having the apple logo its keep restarting ...

Nothing wrong with the system and i did a clean install, i had all her data backup on my iMac

i used the mac os extended (journaled)

oh and its a white macbook, 2 GHz ..

Yes thats in GUID, im thinking putting the hard drive in a external bay and connect it to my mac and try to install it like that and put it back into the macbook when its done ..

i placed the hard drive in the external bay and as it was empty it came up as normal usb external hard drive on my imac, but the problem is i tried to install osx on the external drive while connected to mu imac, everything went fine same as macbook but it froze in the apple logo with the loading animation keep going ....

Im gonna give it one more try intalling it off external hd if it didnt work i take it apple

OK at the end i found out it was a faulty hard drive!

OK so we took it the apple store and they said it either the logic board or the RAM but i was all about the hard drive and stuff, so they said everything will be free to fix it, they took it away and we got a call today saying its ready and guess what was the problem, i was right the hard drive, so if anyone has this problem there is a very high % is the HD, Thanks for the help everyone.