Hi
Try running your update with all background programs/services that are running stopped. Close Security essentials (although your computer may be vulnerable it may be wise to disable your network connections), close programs in your system tray/task manager etc etc...

In control panel type troubleshooter and click troubleshooting, Under System and Security, click Fix problems with Windows Update.

Details of fix can be found here.

Troubleshoot problems installing Service Pack 1 (SP1) for Windows 7 and Windows Server 2008 R2

Hope this helps solve your issue.