I Have this same problem, with the addition that I when I select the not now option on the update, it freezes about 98% of the way through the loading bar. I have a similar issue with Spout and tekkit, though those don't have an issue with me not updating.
Could it be an issue on the minecraft.net end?
Edit: I tried the browser based version and it says that I need to download the java plug-in. I re-downloaded said program twice, and both times it still failed to work with the browser version.
For me, it was AVG. However, lots of people have had this problem, many claiming it was Mojang's end. All I know is that this was the first time I ever had trouble updating, and the same for many people.
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Could it be an issue on the minecraft.net end?
Edit: I tried the browser based version and it says that I need to download the java plug-in. I re-downloaded said program twice, and both times it still failed to work with the browser version.