I'm not too sure what this .net stuff is. It is an option that does not come with Windows, but I have found a few things that require it & have put it on my new PC - the first time it crashed!!!! Incompatable with Windows - WTF Bill
I tried again because I wanted to install digital rights management & "4od", which offers a selection of UK's Channel 4 programming, including for free the last 7 days or so of selected broadcast programs.
It is also required by the Grand Privvy companion (although not any more
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It's a big file, but with broadband and a decent PC it should be worth the effort for the future. It seems better to install it first, rather than let software try and pre-install it on finding it not present (this might be why I had the original crash). Once installed, Windows update will take care of managing fixes and upgrades.