Tiger crashes and on reboot hangs with just the spotlight icon visible. The answer maybe more obvious than you think! Spotight may very well be the culprit. Check out some steps that could help resolve the issue.

In order to rebuild the index, it's easiest to first wipe the main spotlight index and then let the OS rebuild it from scratch. In order to do this, you'll need to boot into safe mode. You can boot into safe mode by holding down the shift key while the computer is booting up.

Ironically, the computer seems to run perfectly fine from safe mode. The default user can log in and do everything that they can normally do, from all appearance. Rebooting normally, without fixing anything, will still give you the same previous sorry state of things, so before we do that...

Open the terminal.app. Once you have terminal.app open, type sudo mdutil -E /. It'll ask for a password, just enter the password for the account you're logged into. If you don't have permission to run sudo, the prompt will bark at you and go back to the prompt. However, you can, alternatively, just log into root. The latest versions of OS X has root disabled by default but, you can enable root through the Netinfo manager.

Open the Netinfo manager, which looks like a proprietary Apple control, but really, it's just a pretty interface to LDAP, which a lot of you are probably pretty comfortable with. Anyways, open the Netinfo manager, click the lock and authenticate. Click the Security option in the menu bar and select "enable root user". It'll ask you to create a root password, please pick a non-trivial, alpha-numeric, mixed-case password. I know you're not gonna do it, but at least I warned you.

Okay, so, if you weren't able to run sudo mdutil -E / before, go back to the terminal and type su to switch to the root user. You'll be prompted to enter the password you just created, do that and when you get control of the prompt again type mdutil -E /.

In 99.9% of the cases, this will fix the problem. Reboot and see if this fixed the problem for you. If this doesn't work, you'll have to do something a little more drastic.

Reboot again into safe mode. After you log in, navigate to your home directory, then go into Library > Preferences and find com.apple.spotlight.plist. Drag the file into the trash and empty the trash. OS X will regenerate the file on reboot and everything will be kosher. If you've been keeping up with upgrades, you might not have com.apple.spotlight.plist, in that case, throw out com.apple.finder.plist, empty and restart.