I don’t think it should be a problem to start from an earlier OS, but others may know better than I. Maybe it depends on what repairs are necessary, I don’t know. As for not being able to start using the disk – if you have an external drive connected via firewire, and it has enough free space, you could install Panther onto that and then choose that as the start-up disk (using “Start-up Disk” in system preferences), repair Tiger as needed, and then re-start with Tiger. Once you get things sorted out, use something like Carbon Copy Cloner (free) to regularily back-up your boot drive for situations like this. Works for me. As always, if you have important, must-have data on the external drive you’re installing to, make a copy of it elsewhere as a precaution. Hope this helps.