Itunes problems

Hello, i have just got an Iphone 4 as a present i have set up the sim card and downloaded the new 9.2 itunes software i have gone through all the steps and its worked fine but when i click on the itunes desktop app it comes up with a message saying :

The folder 'itunes' is on a locked disc and pr you do not have write permissions for this folder.

plzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz help any help would be grately appreciated