Me facing this prob too many times......
even i was nt doin any thing tht time.........
N biggest prob is tht it never happens in front of me....
mean (as it happens rarely).....i was out of my room after locking my comp..N whn i came back..it got restarted ......whn i logged in it showed "your system has recoverd from a serious error"....
wht cause behind it might be....as at tht time no program is running....