If you have suffered the windows update loop step Configuring Update 3 of 3 0%, you will know the limits of frustration.
The standard way of dealing with this is to do a system restore to before the update. From the boot menu, F8 to the boot options and go for repair, then select system restore. Safe mode won't work as that just goes back into the loop.
Recently I had this happen for a second time, only in this case all my restore points had mysteriously disappeared! Quite a catastrophe.
Nevertheless there is a fix. You need to dive into DOS, opt for the Command prompt option, do a change disk (C:) to c-drive then a change directory (CD windows\winsxs)
Don't do a DIR command because you could wait forever if you have a lot in there and you probably will. Instead do a DIR p* to get a list of the files beginning with P. You are looking for pending.xml You can delete it but I prefer to rename it (REN pending.xml old-pending.xml) Then close the command window and reboot, select normal start. You will still come to the loop but this time you will skip through it. Mind you your heart might stop while waiting when the loop first reappears...
You should then get to have your operating system back and your first task is to create new system restore point. Your next task is to go in and turn off MS automatic updates... That may or may not help, I had mine off when the second crash loop happened!
My thanks to Mooh who provided the best explanation that I found on fixing this.
MS has a note about this problem here http://support.microsoft.com/kb/949358
This problem seems to be kicked off by a variety of different updates.