I have come across viruses which prevent you from downloading the latest Java Update this way. Where are you trying to download the update from? Go to the Java site and grab the real one and give it a try.
Then there is something wrong with your internet, your browser, or your system. I just downloaded it fine. I would REALLY suggest you getting Java updated before you worry about playing Minecraft.
Java is incredibly vulnerable. A year old version has so many security flaws on it it's unbelievable.