Start - my computer - R click on C: - select properties - select tools tab - click on checkdisk or check for errors. It will probably want to schedule it for next boot, do so and reboot, let the checkdisk run.
A hard drive on the way out or having some kind of file system corruption can also manifest as you have described.
If its a file system issue this will help. If its a drive problem, get on with the backing up now.
There are a couple of ways to diagnose if it is a heat related problem.
First, run a high activity app like a virus scan or a game. I have some Star Wars games that killed it in minutes.
Second, set your power options to power save mode. If it is a cooling issue, that should prolong its life.
I was positive my problem was a heatsink one, but I seemed to have fixed it without going that far. I am ready, just in case, though. Quite a few people have found dirty heatsinks.