MacBook boots into grey screen

After trying to install Leopard on a 13.3 inch MacBook with an Intel processor , running Tiger, it now boots into a grey screen. The installation failed halfway through and will not load the operating system. I have tried booting into Safe Mode, resetting the PRAM. It can boot from a CD, but the hard drive now thinks it is running 10.5.

It has the right specifications for Leopard, and there was a green arrow on the disk when opened in Finder, which usually means it is ready to go. Being a Intel Mac, I can't even boot into command line mode.

Do I just buy a 10.4 disk and install that? Can I save my data? Can I boot into Mac OS X without reinstalling the OS?

Thanks in advance!

I read that link. So I should just reinstall the OS. No way to boot now.

It should downgrade to 10.4 now, right? Because now it thinks it is on 10.5.

I'll have to buy one because I can't find it.

Thanks for your help.