I am assuming that after you boot there is no way to move away from that notice. If
that is the case turn off the computer. During boot continually hit f 8, which will bring
you into safe mode. See if it works. If so use system restore to get to a point prior
to the update.
This should bring things back to normal.
If you can get to system restore using the normal methods, that would be preferred.
If the above does not work try this Using Last Known Good Configuration
Report back. Hopefully you will have good news for us.