Shutting down laptop months after overheating

I used to sit in bed with my laptop for months but when it started to shut down I stopped doing that. But now, even if I am at a desk (with it unplugged) it powers off. But it will only be unplugged for ten minutes when it shuts off, so it is obviously not out of battery power.
Is this an effect of the former overheating that took place?
(I have a windows vista)