also, if you can open Console (in your Utilities folder), it should give more details about the fatal error that you could post here or send to pro support.

btw--the Apple Info linked in LOTR's post does give info about repairing permissions on 10.1.5 (it's buried in those links and you have to search for it).

Safe boot is for 10.2 and 10.3. For 10.1 the equivalent is fsck (linked above), which means you'll have to reboot and hold down the 's' key to boot into single-user mode to run fsck.