Try this:
1. Press and hold the Shift key as the computer starts up, until "Safe Boot" appears. If this does not permit the computer to start up, a different issue exists.
2. Try removing everything from these folders:
/Library/StartupItems/
/System/Library/Extensions/
3. 1. Mac OS X reviews fonts in the Mac OS 9 System Folder during startup. A damaged Mac OS 9 font file may contribute to this issue. Note: Update to Mac OS X 10.2.4 or later to reduce potenial issues from damaged Mac OS 9 font files.