As some of you know, I had to reinstall Windows a few days ago because it kept freezing almost completely on me, most often during extensive HDD I/O operations. After the reinstall, guess what happened. It started doing it again. It worked fine until I had almost all my programs installed so it can't be HDD failure. So I went into safe mode and started uninstalling everything I could. Guess what, it worked normally after that. So I started installing programs again. At one point before I even reinstalled Windows the first time, I noticed that it would not freeze if I killed the Dropbox process immediately after booting. So this time basically the only thing that I didn't install was Dropbox. And suddenly everything worked. Then I installed Dropbox and my laptop started freezing again. I uninstalled it and everything was fine. Now, I don't really know what exactly the cause was of the problem, but I do know how to fix it: Delete your Dropbox directory. Seriously, do it. Install Dropbox, resync a fresh copy, but don't use the old one. I assume that the hidden directory called ".dropbox.cache" got corrupted or something or that some data was messed up and so Dropbox kept freezing my laptop. Even though the Windows reinstall could have been avoided, I'm glad that I did it, because I've been putting it off for weeks (not counting my 2 months while I was "out of order").
So if you experience the same problems with your PC freezing up and you have to do a hard restart (CTRL+SHIFT+ESC to bring up the Task Manager takes a lot of time, even up to several minutes if it even shows up) and you are using Dropbox, consider this solution. Once again, uninstall Dropbox, delete your Dropbox directory, install Dropbox again and let it download all files from the cloud again. Obviously if you have a lot of data up there (like in my case with around 8 GB right now), then it might take a while.
Dude you have no clue how much this will probably help me. I'm on a mac, but I had the same sudden high HD load out of nowhere, and it kept freezing everything, and this went through two harddrives.. so let's hope that's what it was for me too.. I realized that the only thing it always had in common was dropbox.
My computer just started freezing up too. I stopped Dropbox and deleted the folder, I'll see what happens.
EDIT: seems to have done the trick.
It's likely that prolonged usage and syncing with Dropbox causes this. I've been using Dropbox since version 1.0.x and now it's 1.4.7. So it's either a general bug, a bug from an earlier version or a problem when updating versions.