hello,
it happens quite frequently that visual studio complains about this:
For me the offered target pack is not there when trying to install it.
So its only install able with a windows version and manual dism or windows update. Its still some extra work and might fail in some cases.
Unity with vs needs .net 3.5
It would be great and avoid much confusion if unity would install it.
Well as i know it might not work for the webplayer or something else, it is not the default setting and i think unity and vsstudio should work out of the box without extra installations.
So if it is now working completely on 4.5 change the default settings to 4.5 or include 3.5