Do you have McAfee. If so, you must completely remove it, run a McAfee removal tool, update the computer and then reinstall McAfee.
This happened to me and after a month of go around with Microsoft, including telling me to completely remove everything from my computer and reinstall the os and programs, I finally got a head tech who had me do the above. It also might be a problem with other firewall and anti-virus program.
Have 2 year old computer running Vista....will not update....never has...get error message...MS fix they gave me does not work??? Thanks