Desktop Problem

For the last week or so I have been dealing with a sudden change with my Desktop. When I try to move any file onto or off of it, it brings up a box asking me to "Authenticate" so I type in my password and it accepts and moves the file to the Desktop.
When I try to download any file to the Desktop (which is my default download destination) the file won't even start to download and I see the message "Cannot creat file".
If I plug in my flash drive I can download directly to that and I can also move files freely between any folders, other than my Desktop.
I must have done something, but I have no idea what. I've scoured the net looking for someone with the same problem, but no dice.
I have a MacBook Pro Intel 2.33. I'd rather not have to re-format the drive since it's only 2 moths old.
Anybody out there with any thoughts as to how I might fix this would be greatly appreciated.

My neighbor had this problem. Click on the desktop background image. Get info (command+i). Reset the ownership & permissions so that you can read & write. You can probably also run repair permissions from disk utilities.