On a Vista Home Premium machine, while attempting to download and install Security Update For Microsoft XML Core Services 4.0 Service Pack 2 (KB936181) you may receive a failure each time with error code 643 and 8000FFFF
For Windows Vista Users
Go to Start > Settings > Control Panel and select Programs and Features. Then Uninstall all entrys related to Microsoft XML core services (These will be clearly marked)
Then close everything down back to the desktop and do a search for updates and you should get the normal 1 XML Core Services 4.0 update. Install this and it
should fix the problem.
For windows xp users
MAKE SURE YOU UNINSTALL THE UPDATE FROM YOUR PROGRAM FILES
Now you need to download the update program “msxml4-KB936181-enu.exe” from here and install.
First ran the program and told it to remove all and reboot then ran the program again to install it.
Then ran the Windows Update file for “Security Update for Microsoft XML Core Services 4.0 Service Pack 2″.
After installation you need to reboot your machine