Sounds like the problem my teacher had with his Powerbook. It could be many different reasons why it won't install:
1. Its a burned copy of Tiger or a copy of Tiger that came from a PowerMac G5 - these won't work. They have special coding so that you can't just install it on whatever you want.
2. You are having logic board problems - logic board problems will affect whether or not it will install.
3. Bad drive(s) - very rare, but still may not install. - Usually related to logic board.
4. Processor upgrade cards - Tiger will not install with certain upgrade cards.
5. Bad RAM - RAM is very important, try removing any new ram that has been installed.
6. Not enough hard drive space to install or needs to be formatted. I'm looking more towards #1 or #6.