Installing Leopard Help, Won't install..

Hey Guys,
I have a little problem with installing Leopard. I hope someone can help?
Well the problem is, it just won't install. I have a Early MacBook, one of the earlier versions.
Spec's:
1.83 ghz
1GB Ram
Combo Drive (so i can read the dvd)
and I have enough hardrive space (i have 13 gigs)

and I don't get it. Everytime I try to install, it doesn't get anywhere. I open the installer, and then it tells me to Restart, right? and then I do. after 2 minutes, it comes up and asks me which language. And then after that, it just says that 'It can't be installed on this Computer'.
I have all the system requirements so I don't get it..
I will add that this isn't the real leopard DVD that you get from the retail. My friend bought a New Macbook and i'm using that 'Mac OS X install Disc 1' to install. Its from a different macbook so it should work... Could anyone please help I really want this!!

Thanks guys!