Problems
See also: Isolating issues in Mac OS X at support.apple.com

Strange behavior:
Crete a new account in "System preferences" > Accounts : Click on "+"
Login to the new account.  If no problem here it is probably something in your
user Library folder.

Try an archive and reinstall.

This worked to fix a problem where multiple finger track pad gestures didn't
work on my account after I installed files from my old mac with Migration Asst.
However you may need to reinstall without restoring user setting which may
cause other problems. See Preference Restoration.

Can't see servers on OS X 10.4
Turn off Firewall in: "System Preferences" > Sharing > Firewall

Startup problems

- Boot to "Safe Mode" (Safe Boot) by holding the shift key from chime until
you see "Safe-boot" on the progress bar box.  This disables a lot of functions
(kernel extensions) and runs a fsck in background.

-  Boot to single user mode. Hold Cmd-S at startup.  You can then run fsck or fsck -f.

-  Select System by holding optn at startup.

See Magical Macintosh Key Sequences

Hardware test- Insert Applications Install Disc 2 (Intel macs shipping with 10.5.5 or later) Restart your computer, holding down the “D” key

See:

How To Troubleshoot Your Mac

Intel vs Power PC Processors.
 Around 2006-2007 Apple switched from Power PC to Intel processors in new Macs.
 
 Developers were encouraged to write "Universal Binary" applications which had
 native code for both Intel and Power PC processors.
 
 They also included software called "rosetta" in intel Macs which would 
 translate Power PC code to Intel code.
 

 According to Apple, applications with heavy user interaction but
 low computational needs (such as word processors) are well suited
 to translation via Rosetta, while applications with high computational needs
 (such as raytracers or Photoshop) are not.

See also:

Apple Tech Exchange: images.apple.com/support/
MacFixIt troubleshooting solutions for the Macintosh:www.macfixit.com
http://www.missingmanuals.com
Mac Resource Page: www.macresource.pair.com
MacCentral Macintosh users news and forums:www.maccentral.com

How To Troubleshoot Your Mac

Restoring User Library Files 

last updated 6 Feb 2009