"Applications" Cannot Be Modified?

Hey Everyone,
I am having a strange problem with my Applications folder. Everytime I try to copy an app or file to my "Applications" folder, I get a message saying the "Applications" folder could not be modified, and then I have to Authenticate it by putting in my password. I was wondering why it is doing this. I am the admin of my Mac. It only started doing this within the last few hours.
Thanks for any help.