Could not install package 'Xamarin.Forms 1.0.6186' (Windows)

I installed Xamarin from the installer on the homepage onto a Windows 7 Pro with SP1 (32-bit) PC on 12/06/2014. The t-shirt sample app builds and runs as expected.

I want to create my own app from scratch, so I am following the instructions here:
http://developer.xamarin.com/guides/cross-platform/xamarin-forms/introduction-to-xamarin-forms/

I created a new 'Blank App (Xamarin.Forms Portable)' solution in Xamarin Studio. As soon as it opens I get 'Could not add packages. Please see Package Console for deails.' in the top status bar, and the following in the 'Package Console' panel:

Adding Xamarin.Forms...
Installing 'Xamarin.Forms 1.0.6186'.
Added file 'Xamarin.Forms.Build.Tasks.dll' to folder 'Xamarin.Forms.1.0.6186\build\portable-win+net45+wp80+MonoAndroid10+MonoTouch10'.
Added file 'Xamarin.Forms.targets' to folder 'Xamarin.Forms.1.0.6186\build\portable-win+net45+wp80+MonoAndroid10+MonoTouch10'.
Added file 'Xamarin.Forms.Core.dll' to folder 'Xamarin.Forms.1.0.6186\lib\MonoAndroid10'.
Added file 'Xamarin.Forms.Core.xml' to folder 'Xamarin.Forms.1.0.6186\lib\MonoAndroid10'.
Added file 'Xamarin.Forms.Platform.Android.dll' to folder 'Xamarin.Forms.1.0.6186\lib\MonoAndroid10'.
Added file 'Xamarin.Forms.Xaml.dll' to folder 'Xamarin.Forms.1.0.6186\lib\MonoAndroid10'.
Added file 'Xamarin.Forms.Xaml.xml' to folder 'Xamarin.Forms.1.0.6186\lib\MonoAndroid10'.
Added file 'Xamarin.Forms.Core.dll' to folder 'Xamarin.Forms.1.0.6186\lib\MonoTouch10'.
Added file 'Xamarin.Forms.Core.xml' to folder 'Xamarin.Forms.1.0.6186\lib\MonoTouch10'.
Added file 'Xamarin.Forms.Platform.iOS.dll' to folder 'Xamarin.Forms.1.0.6186\lib\MonoTouch10'.
Added file 'Xamarin.Forms.Xaml.dll' to folder 'Xamarin.Forms.1.0.6186\lib\MonoTouch10'.
Added file 'Xamarin.Forms.Xaml.xml' to folder 'Xamarin.Forms.1.0.6186\lib\MonoTouch10'.
Added file 'Xamarin.Forms.Core.dll' to folder 'Xamarin.Forms.1.0.6186\lib\portable-win+net45+wp80+MonoAndroid10+MonoTouch10'.
Added file 'Xamarin.Forms.Core.xml' to folder 'Xamarin.Forms.1.0.6186\lib\portable-win+net45+wp80+MonoAndroid10+MonoTouch10'.
Added file 'Xamarin.Forms.Xaml.dll' to folder 'Xamarin.Forms.1.0.6186\lib\portable-win+net45+wp80+MonoAndroid10+MonoTouch10'.
Added file 'Xamarin.Forms.Xaml.xml' to folder 'Xamarin.Forms.1.0.6186\lib\portable-win+net45+wp80+MonoAndroid10+MonoTouch10'.
Added file 'Xamarin.Forms.Core.dll' to folder 'Xamarin.Forms.1.0.6186\lib\WP80'.
Added file 'Xamarin.Forms.Core.xml' to folder 'Xamarin.Forms.1.0.6186\lib\WP80'.
Added file 'Xamarin.Forms.Platform.WP8.dll' to folder 'Xamarin.Forms.1.0.6186\lib\WP80'.
Added file 'Xamarin.Forms.Xaml.dll' to folder 'Xamarin.Forms.1.0.6186\lib\WP80'.
Added file 'Xamarin.Forms.Xaml.xml' to folder 'Xamarin.Forms.1.0.6186\lib\WP80'.
Added file 'Xamarin.Forms.1.0.6186.nupkg' to folder 'Xamarin.Forms.1.0.6186'.
Successfully installed 'Xamarin.Forms 1.0.6186'.
Adding 'Xamarin.Forms 1.0.6186' to XamarinFormsPoc.
Could not install package 'Xamarin.Forms 1.0.6186'. You are trying to install this package into a project that targets 'portable-Profile78', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

When I build I get the errors in the attached screenshot. Bear in mind I haven't modified anything yet - this is a fresh solution.

These two forum posts have the same problem, but for Mac OS X not Windows 7:

• http://forums.xamarin.com/discussion/17325/mac-framework-not-installed-netportable
• http://forums.xamarin.com/discussion/17315/build-problems

I have tried the following things:

1. Closing and re-opening the solution, as suggested by JasonASmith in one of those posts.
2. Reboot.
3. Installed Mono. The solution for the two posts above seems to have been to install the Mono MDK from http://www.go-mono.com/mono-downloads/download.html. That page only lists a general Mono installer for Windows, not an MDK. There is no mention anywhere I can find on the web of a Mono MDK for Windows, so I ran the full installer hoping it would include the necessary pieces. I recreated the solution as above, but still the same issue.
4. Verified I have Microsoft .NET Farmework 4.5 installed (I ran it's installer and it said it was already installed).

I've attached the text from the Xamarin Studio > Help > About > Show Details dialog.

Anyone have any ideas on what I need to do to get this running please?

Thanks
Seb