double click won't open files

Background:

Complete insides and keyboard of MacBook w/OS 10.6.8 were replaced under AppleCare 1 year ago after 1 year of operating grief. Just out of Applecare April 30, so Apple Techies will not talk to me.

Problem / Symptom:

Macbook w OS 10.6.8. Double clicking files and folders at all levels will not open them. I must highlight the file/folder & open same from File Menu. Works but painful. Yet strangely Alias links in DOC often work just fine?? Once in an application, double clicking usually works - within that application, not for external file/folder access. Also Mac has slowed to unacceptable level and I am faced with the coloured "wheel of Death" for an etarnity & then often the action is never completed. Attempts to open links in Safari very often freeze - for sites I can easily access from my wife's iMac.

My Actions to date:

Have moved virtually all files / folders from desktop to Data Folder under Hard drive icon. Have Repaired Disk & Permissions several times w/ Disk Tools. Have shut down and restarted Macbook too many times to count. Have rebooted modem and Time Machine. Have Zapped PRAM,

Over 1 month, speed has dropped to a crawl. Takes ages to complete any action call

I welcome the community's experience and advice. HELP!