For Windows Vista, 7, and 8:
1.Completely exit Steam
2.Right-click on the shortcut which you use to launch Steam
3.Select "Run as Administrator."
For Windows XP:
1.Exit Steam completely.
2.Go to the "Start" menu, select "Run", type "regedit" and click "OK".
3.Navigate to the following registry key: HKEY_CURRENT_USERSOFTWAREVALVE
4.Right click and select "Permissions"
5.Check the "Allow" boxes for "Admin" and "System" for both "Full control" and "Read".