How can I cancel a stalled upgrade?

When I shut down OS 10.9.2, I get a window that asks do i want to continue or abort the upgrade. It gives no clue about what is getting upgraded.

When I choose "continue", I get a very short (in time) view that a file is missing. It shows me the file, but it shows it very quickly and I don't have the time to write it down. Now I am beginning to wonder if I have some sort of malware in there. Is there a way to just permanently cancel this so-called upgrade?

THanks,

Jim