I cannot install an application in the Applications folder without removing it first

Question

For some time now, I have noticed that I cannot install an application into the application folder without first removing it from the folder. If I try to install it by dragging the icon to the application folder, I get a warning that the application can't be moved because the folder "Applications" can't be modified. I click on authenticate and enter the root username and password. I am warned that the application already exists in the location. I am asked if I want to replace the item. The options presented are "Keep both files", "Stop", and "Replace". I choose "Replace". I am then prompted for an administrator's name and password. I enter "root" and the root password. I then get this lengthy error message:
The operation can’t be completed because some items had to be skipped. For each item, choose File > Get Info, make sure “Locked” is deselected, and then check the Sharing & Permissions section. When you are sure the items are unlocked and not designated as Read Only or No Access, try again.
I checked the permissions and "locked" was already deselected. The permissions grant my regular user read and write permissions, "staff" read-only, and "everyone" read-only. If I add "admin" with read and write permissions, nothing changes, i.e. I get the same error. If I try to enter my user credentials, they are rejected. My user is not an admin user. Previously, when I enterered the root username and password, the operation would complete successfully.
Now if I first remove the application from the Applications folder, then drag the application to the folder, I am prompted for an administrator's name and password. When I enter it, the application is copied successfully.
I have tried fixing file and directory permissions from the disk utility, but that doesn't fix the problem.

Answer 1

When you say, "locked" was already deselected, I take it that you mean the small icon at the bottom right of the Get Info window displayed an open lock picture.
Have you tried locking it, then unlocking it again?  I expect you will be able to lock it by simply clicking on the open lock icon, but to unlock it again you will need to supply an Admin password.
Odd that DU didn't fix it.  That would be my first thought also.  Suggest trying it again, if you haven't already done so.
Speaking of DU, have you verified your HDD?
My Applications folder permissions read -
System          Read and Write
admin           Read and Write
everyone      Read only
Message was edited by: El Deanio