Let's celebrate! My PC is up to speed again.
There was nothing to do with Windows Vista.
We have been bashing and blaming it for nothing.
The cause for the slow down was the missing USB2.
Or rather missing USB2 enhanced host controller.
Discovered this when I plugged in an external drive.
The PC reported it cannot operate at optimum speed.
It said the USB 2.0 hi speed port was not present.
This caused the drive to work in slow speed USB1.
It's a pain to edit video at this old speed.
Especially when editing high definition videos.
Response was so sluggish like a x386 computer.
Did a few things before pin-pointing the actual cause.
Cleaned desktop, defragmented, scanned for virus and spyware.
Cleaned registry, system restore and asked for help.
Finally I found this forum thread that ended it all...
Here's how to bring back the missing USB2:
1. Turn on PC. Press DEL. Get into the BIOS screen.
2. Set to factory default settings.
3. Save and exit.
That's it! 3 days of agony solved in 3 steps (less than 1 minute).
Thanks to the internet for the free information.
Thanks to my facebook and twitter friends giving suggestions.
Thanks to Dr Computer, Stewart Loh, for his professional advice.