Can't update Xcode 4.2. Error: please consult var/log/install.log for more details?

Question

UPDATE: Since this question is getting some views, I figured I'd better highlight the fact that I solved the problem by simply re-downloading Xcode from the Mac App Store. Upon re-installing it even gave me a handy option to delete the previous version and replace it with the new one. Hopefully this helps everyone out there with the same problem! :)

So I'm trying to update my Xcode 4.2 which I downloaded from the app store, but I get this error when I use the 'Install Xcode.app' updater:

Anyone know what I'm doing wrong or how to fix it?

I'm not really sure what other information I should provide, so please just let me know if you want more info :)

Edit: Here is the full install.log (I think): http://pastebin.com/GTzkpL3X

This is an excerpt that actually shows the error:

Failed install preflight: Error Domain=PKInstallErrorDomain Code=102 "The package “DeveloperTools.pkg” is untrusted." UserInfo=0x7f9d5da29060 {NSLocalizedDescription=The package “DeveloperTools.pkg” is untrusted., NSURL=../Packages/DeveloperTools.pkg -- file://localhost/Applications/Install%20Xcode.app/Contents/Resources/Xcode.mpkg/, PKInstallPackageIdentifier=com.apple.pkg.DeveloperToolsLeo, NSUnderlyingError=0x7f9d5d9eab40 "The operation couldn’t be completed. CSSMERR_TP_CERT_EXPIRED"}