Begin with:
Mac OS X: Starting up in Safe Mode - http://support.apple.com/kb/HT1455
What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564
Safe Boot takes longer than normal startup - http://support.apple.com/kb/TS1884
Mac OS X 10.4, 10.5 - Computer shuts down during Safe Boot - http://support.apple.com/kb/TA24054
Safe boot mode runs a directory check command similar to that used by Disk Utility's repair. In OSX 10.4 and later it ignores some stored information (cache) that is normally read that speeds up the boot process, and it moves some caches to the trash. It also uses only System fonts and disables all Startup Items, third party items, and any Login Items. Networking and some video components may also be disabled.
Also worth installing on an older computer is Applejack:
http://applejack.sourceforge.net/
Read the instructions on how to use it because it's a bit unusual (needs to be done in single user mode).