[brihas]

I had the same problem, and called Apple support. They said they have been getting calls about it all day. The solution is as follows:

Turn off your computer (don't worry it is safe to do so).

Restart and log in.

Go to /Library/Updates/ and delete the Mac OSX Update folder.

Use the standalone installer to update your system. You can find it at http://support.apple.com/downloads/