This just happened to me yesterday and now it did a few moments ago. I returned to my PC and began to resume what i was working on. I had a few tabs open (Facebook, Tek Syndicate and some pages for school), I open a new tab and everything goes to hell.
The PC just completely freezes. The mouse still moves but everything, even ctrl+alt+del, doesn't work. I have to do a hard reset in order to get the PC back and then it takes an obnoxiously long time to start up. Sometimes over 4 min, which is intolerable when the normal start up time is around 45 seconds.
I've run HD Tune to check my HDD for errors but nothing was found. SMART is all reporting back well too.