I've had the exact same problem. Log on, then after a short period of time, everything freezes except the mouse. Cannot reboot with Icon Button clicks or Ctrl/Alt/Delete. Need to manually hold the power switch/button.

I ran the following types programs:
SpyBot: Nothing
AVG: Nothing
Registry Mechanic: Nothing
Malwarebytes: Nothing
Defragged: Nothing
Memory Tests: Nothing
Check Disk: Nothing

Then I checked for updates and found a Windows Update that was loaded on September 5th, 2011. I have uninstalled that update, and am waiting to see if the problem continues to present itself. So far, I am able to write this message.

If anyone has insight of what else I can check, and links to freeware that can help diagnose the cause, please let me know.

Is the problem persisting since you got rid of the update? i would say that its the most likely cause as vista especially has had freezing issues with updates. Have you installed anything else recently since the freezing started happening? how many/what are your start up programs(easily found by running msconfig).