IMac Frozen randomly for no obvious reason

Question

My iMac has been acting up weird since two accidental power-off 2 weeks ago. The power off was due to a plug fail at my old house and it happened 2 times in 10 minutes. I eventually fixed the plug and made sure that never happened again. But the 2nd power off looked damaging to my iMac since it triggered automatic safe boot, which looked daunting to me.
Now the problems is that my iMac would randomly freeze for no reason when I'm only running an iTunes (paused), IM program and 2 browsers with about 6 gb hard drive space on a iMac 20 inch model with 2.66 ghz CPU.
It doesn't make sense because I only have some web pages and itunes open(itunes not even playing anything), it shouldn't consume so much processing power that it freezes the machine over.
Random freeze means that sometimes it freeze on login, sometimes it doesn't. Sometimes it doesn't freeze for a day but suddenly it freezes next afternoon.
I have one new app installed for anti-virus, the McAfee, not sure if the firewall is doing anything. Also I have my mac's firewall on as well.
Here are the stuffs I tried so far:
1. Restart, boot from CD, repair disk and repair permissions.
2. Restart, reset PRAM
3. Install the snowleopard again on the same disk so it overwrites everything and i update the system once again to the latest version.
4. The mouse is working during the freeze because I can see it moving, just not other process can properly run.
Please let me know if there is any log file on the mac that I can access to further diagnose if this is a software failure or hardware failure. I certainly hope i don't have to replace harddrive and memory for this.

Answer 1

Your computer is a mess probably from a combination of things. I would start first by following McAfee's instructions for uninstalling their software for several reasons. First it's of no value because there are no, none, zero, zip virus's for  OS X currently. Next is most antivirus applications create  more problems than they solve and third they slow down a computer by taking valuable system resources.
Next I would invest in a UPS, this will help in the event you have power problems again. A UPS allows you to do an orderly shut down in the event of a power failure or major power spike.
After  you have done both do a SMC reset, Repair Disk Permissions (again) and restart in Safe Mode to clear some caches. If it still is unstable you will need to bring it into your local Apple Store or AASP to be checked out.
SMC RESET
Shut down the computer.
Unplug the computer's power cord and all peripherals.
Press and hold the power button for 5 seconds.
Release the power button.
Attach the computers power cable.
Press the power button to turn on the computer.
PRAM RESET
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.
Roger