PowerPC :: How To Change Admin Password On G5

I have inherited my uncle's computers and I'm having problems logging into his iMac G5 20". I have the original Tiger Install DVD, I put it in, restart, pressing C, etc., but the install DVD doesn't run and I just end up at the login screen. He also had an iBook G4 and I used the same trick to change the admin password - it worked just fine. Does anyone know why pressing C isn't starting up the install disk? Its the original.

ALSO: I've read of a workaround using the single-user password reset option that goes as follows:
- start computer pressing Command-S
- sh /etc/rc
- password 'Uncle Paul'
- enter a new password twice
- reboot

I'm afraid to do it though, because I need to gain access to my uncle's files before I reinstall. My parents insist of getting all his pictures and stuff (he was also writing his autobiography.) Will doing the above Command-S measure mess with any of his files?