An AVG scan turns the computer off

Hi Rob,
In order to analyze it please provide us with diagnostic output (run it after inducing described behavior) using this tool and please add detailed description of issue.
Thanks

I'm using AVG free & have been for some time, the computer is running windows Vista premium.
I tell the computer to do a whole computer scan and after a while the computer turns off, when I turn it on it goes to the safe screen (I have choices) and the AVG log shows no record of the scan.

I have successfully used Trend Micro Housecall with nothing found.

Anyone seen this before?

Hi Rob,
In order to analyze it please provide us with diagnostic output (run it after inducing described behavior) using this tool and please add detailed description of issue.
Thanks