I had quite a scare a few hours ago. I was browsing my Mac's HD when I chanced upon a folder named "usr" (it's on the first window you see when you click on your HD [joined by the Library folder, System folder, Applications folder, etc.]). I thought to myself, "Hey, I didn't create this." so I decided to delete it. BANG. All of a sudden Safari hangs, and so do other the other apps. I restarted my Mac but it couldn't get past the swirling thing. A message kept popping up saying that I needed to restart my Mac. I tried restarting; it couldn't. I had access to Windows but that was all. So, I reinstalled (choosing "Archive and Install" so as not to delete existing files and settings). Voila! It was fixed. I did a research about the usr folder and it seems like a lot of people have also accidentally deleted theirs. I also read that the folder was supposed to be invisible, I don't know what I did to make it appear.