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