Enable preview in Visual Studio 2012 after Update 1
After installing Visual Studio 2012 Update 1, my Visual Studio stopped opening files in preview mode when I clicked them in the Solution Explorer. This setting seems to have changed with the installation of Update 1. To re-enable opening files clicked (once) in the Solution Explorer in Preview mode, go to ‘Tools’ – ‘Options’ – ‘Environment’ – ‘Tabs and Windows’.
There, check the box under ‘Preview Tab’ – ‘Single-click opens files in the preview tab in:’ – ‘Solution Explorer (Alt + click to avoid previewing)’.
Hope this helps
This is a lot easier: the 'Preview Selected Items' button in the Solution Explorer toolbar: