Please follow the steps :
http://support.microsoft.com/kb/957307
To resolve this issue, stop and then restart the Windows Update Service. To do this, follow the steps that are appropriate for your operating system.
Follow these steps:
1. Close any Windows Update or Microsoft Update browser windows.
2. Click Start, click Run, type cmd, and then click OK.
3. Type the following commands. Press ENTER after each command:
net stop wuauserv
net start wuauserv
4. Close the Command prompt window.
5. Try to install updates again.
Windows Vista and Windows Server 2008
Follow these steps:
1. Close any Windows Update or Microsoft Update browser windows.
2. Click Start , type Command Prompt, in the list of results, right-click Command Prompt, and then click Run as Administrator.
If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3. Type the following commands. Press ENTER after each command:
net stop wuauserv
net start wuauserv
4. Close the Command prompt window.
5. Try to install updates again.