Hi joel...Welcome to D-A-L
For your Dimension 4100 - System Codes and Messages - Diagnostic Codes, Post Beep Codes, and System Messages...
Documentation
Dell's also have a series if 4 Diagnostic lights usually on the back (some of our newer Dell's have them on the front). They will give a code (ie 2-Green lights, 2-Yellow lights) if issues. If no issues all 4 lights are always Green. If any are Yellow then there is an issue.
Edit: ***dobhar shakes head as he just noticed that this thread was actually started back in Oct 2008***