laptop shuts down-Vista

I have Vista and for an unknown reason the laptop shuts itself down, sometimes
after 45 mins, 30 mins, sometimes after 10 mins. not know why it happen...
I have tried system restore, downloaded windows updates, updated avira and
done a full scan (of which took several hours and the laptop stayed on) all fans
seem to be working.
Nothing seems to trigger it in particular and timings are different.
Any ideas would be welcome.