Problem updating Xcode to 4.6

I'm having a problem updating Xcode to the latest version. It seems to download OK from the App Store but when it starts to install it my iMac shuts down. I've tried this several times and the same thing happens every time. I see that in Finder there are two entries for Xcode, one being the App Store download. I saw a suggestion when someone was having problems with an earlier upgrade that you should try to see if the App Store would allow the new Xcode to be downloaded again but it only gives me the option of downloading the upgrade.

How can I sort this out?