Solution 1 -- Just reboot your computer and log in using an account that has the necessary administrator privileges and your problems could be solved as easy as that. If you still have trouble, then try solution 2, below. If you are on a Local Area Network you may need to get authorization from your system administrator to access the file.
Solution 2 -- The problem is probably a vital Windows service that you need to share files and folders has stopped and you simply need to re-start it. In order to re-start it follow the steps below and hopefully this will take care of any problems and you will be able to get back to work. This is likely to happen with programs like the print spooler or files in your C:\Users\<user>\desktop folder (especially if you have upgraded from Windows XP to Vista or Windows 7). If you are upgrading from XP then your files will inherit the permissions given them in XP. If you want to change that you'll have to use your admin privileges to give you full control in Vista.

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Start the process by clicking Start, then right-click Computer (this will be My Computer for Windows XP), and finally click Manage. You will see Services and Applications displayed on the page, just click services, and then scroll down the page until you find the Server entry. Double-click Server. Select Automatic on the Startup Type down-drop menu that appears, and then the Start button or the button that appears. Finally, just click Apply and Ok, and you should be able to open, move, or copy the file or folder you were having problems with and sharing work should work again.