VS 2010 can be installed “side by side” with previous versions of Visual Studio.  What this means is that you can install it on the same machine as VS 2008/VS 2005/VS 2003 – and use all of the versions at the same time if you’d like.

.NET 4.0 can also be installed “side by side” with previous versions of .NET on the same machine.  .NET 4.0 has a new version number for both the framework libraries and CLR engine – which means it runs completely independently from .NET 2.0, 3.0 and 3.5.  What this means is that you can install .NET 4.0 on a machine that has .NET 2.0/3.0/3.5 installed, and configure some applications to run using .NET 4.0 and others to run using the older .NET versions (the IIS admin tool allows you to configure this for ASP.NET applications).  This allows you to use .NET 4.0 for new applications - without having to necessarily test and upgrade all your existing ones.