nikkey

I am trying to upgrade from 10.3 to 10.4, and I'm pretty new to Mac so I'm not 100% what is going on, but when I try to boot off the DVD I can hear it being read but then I just come back to the normal logon as if it didn't boot from the DVD. I have also tried the optional installation and it reads all the packages and then a prompt appears saying it needs to check a program before it can install. I then get an error message saying Mail cannot be installed.

Can someone please help me cause I am very lost...