Windows has failed to open Security Options Dialog

I feel I can add to the ever increasing posts
on this error.
I have Vista Home Premium. A 500MB Seagate Barracuda 7200-10
There is another code to go with this:ST3500063 0AS Disk Device.
The main board is ASUS P5N-E SLI nForce 650i SLI(C55)
I have nVidia GE Force 8500 GT display
I have found that some legacy software which worked properly
on WINDOWS XP seemed to undermine my Vista.
I have eliminated these, yet still have generated the error.
The symptoms match the others. I am certain it is the same
error.
If we can find similar hardware, perhaps it will narrow down.
Windows reports no errors after defrag.
I have looked in many places, but the advice never points to the cause or the correct cure.
My experience is that I can usually recover after switching off
(not on standby)and waiting several hours.
The computer shows no sign of overheating.
Occasionally I need the rescue disk, but this works only
after a long wait, as does Safe Mode.
On other occasions only a complete reinstallation will rescue
the computer.

The antivirus is now McAfee.
I have tried many times to check for malware and virus with
different packages.
My problems have been occurring over 18 months.

Theo

I was wondering if the problem could be caused by a
poor quality Power supply.
If there is anyone out there still struggling, would they post
the name of maker and part description.
Mine is HEC 550TD-PTE.

I have read that it is not one of the better known suppliers.
If my setup of hardware is near it's 550 maximum then
maybe this is the problem.
?
Theo