A version of .NET is included in every Windows installation so you could say it is a part of the OS. If you have an ATI/AMD graphics card, you can't update your drivers without having at least .NET 2.0. An ever increasing amount of other software is employing the framework as well. It has reached the point where you could say it is practically essential.
I have never had a problem with a .NET update since it first came out. My only complaint is that sometimes the updating takes a very long time to complete, especially on older machines.
I agree with DavidR, with one exception: XP does not come with dotnet unless you have a program that needs it and you put it there, or that program automatically installs that specific dotnet on installation, but Win 7 comes with .NET 2.0, 3.0, 3.5 installed already. As far as I know, .NET 1.0 is still optional, as is .NET 4.0. We're talking
native dotnet applications supported by the newer os's whereas they weren't before. Vista may be the same as Win 7, but I do not know that. It's Microsoft that is building their os's to run these from the get-go now. From what I understand, removing these native dotnets may cause problems
that may or may not be recoverable in that os.