Had exactly the same problem with my wife's machine (XP pro with SP2) I had selected automatic updates on on it and the other night it spent 12 hours trying to complete the first installation of 2. Eventually I switched it off with an extended push on the power button and the machine then booted up fine, however, the same problem occurred when I tried to turn it off again! I found the cure to be to restore the system to a date before the update occurred, turn off automatic updates, then visit the microsoft update site to get them and manually hit the button to install them. System now working normally again, but she is now notified when updates have been downloaded and are ready for installation. Then I get summoned!
As my (similar) machine only downloads updates without automatically installing them, I had no trouble.
P.P.