computer shuts down
 

A couple of weeks ago I reported that my PC shuts down. It is still happening, but now to my other PC as well.
I appears to mainly happen when, when you go to a secure site - internet banking, ebay, paypal etc. The moment you start typing in account codes it shuts down. It also does it when filling out questionnaires.
I have run AVG anti-virus and anti-spyware, Avast anti-virus, Ad-aware, AFT-cleaner, Hijack this, Malbytes anti-malware, Registry Mechanic, Spybot Search and Destroy, all to no avail.
The problem is still there.

I originally thought the problem was hardware, so bought a new m-board, dual core P4 processor, new ram etc. Did not sort out problem.

I am now at my wits end.
PLEASE is there anyone out there who can help.