So today has been a pretty interesting day for me as a mac owner.. Earlier today I was on my computer and in finder under the HD there was this new file called "usr" I thought it was some random file that got added when I downloaded this thing for my school the previous night and I deleted it. Well it crashed my computer... I called apple and they told me that is a system file that is supposed to remain hidden, but for some unknown reason it stopped being hidden. They told me to go to a store and a genius would reload the OS. So, that's what I did... to make a long story short the genius told me he could reload the OS without erasing the hd so I could keep all my stuff that was on it (documents, movies, pictures, etc.) it worked and the computer is back up and running... but now I see that in finder under the HD there is something called "Previous Systems" now i'd like to delete it because it's 11gb but i'm afraid to for obvious reasons... when I moved it to the trash it came up saying it needed me to type in my password to delete it, which is what the other thing said right before i crashed my computer... so can I delete it and be ok? Thanks for your help!