Hi,

Let's see:
* The computer actually resets, so it's not a monitor issue (except if it's a laptop).
* There are no error messages afterwards, so it probably did go through shut down of its own; it's likely not a power problem.
* There are no error messages before, so it's probably not out of disk space. (It's still is a good thing to check the hard disk, later, as when you unintentionally filled it you may have fragmented disk space, which would mean disk access may have become slower.)
* If it really does boot each time (Check that you see the start-up screens.), it's not power management.
* I, myself, am not near. (I'm rather sensitive to electric current, and judging by the weird behaviour of some machines, the feeling in mutual.)

Generally having a classroom full of computers to keep an eye on, I see things like this most weeks. There can be all kinds of curious causes, but the most common by far is overheating. Most often, this can be remedied by opening the machine and clearing the dust out (All the dust: if necessary, take out the power supply and vacuum its inlets and outlets.)

Block of experience, gained the hard way:
If you remove dust, try to use a vacuum cleaner, or a brush. If possible: Don't wipe with cloth as the friction can build-up electrical charges large enough to fry a chip. (Do check for brush hairs afterwards.) If you're the kind of person that always has sparks flying when dressing in synthetics: make sure while working inside the computer you touch metal regularly to discharge, again to protect the chips. Before you open the computer for this, disconnect it from the net completely (that means all cables, even the cables to the monitor and the modem etc. that are themselves connected to the power). And, of course, make a back-up, even of that one file you just downloaded or wrote, because recreating it may turn out to be harder than you expected.

I hope this helps with the Gremlin hunt.

                                                                Mysha