If you have installed a previous version of VS 2010 on your machine (either the beta or the RC) you must first uninstall it before installing the final VS 2010 release.  I also recommend uninstalling .NET 4 betas (including both the client and full .NET 4 installs) as well as the other installs that come with VS 2010 (e.g. ASP.NET MVC 2 preview builds, etc).  The uninstalls of the betas/RCs will clean up all the old state on your machine – after which you can install the final VS 2010 version and should have everything just work (this is what I’ve done on all of my machines and I haven’t had any problems).

The VS 2010 and .NET 4 installs add a bunch of new managed assemblies to your machine.  Some of these will be "NGEN'd" to native code during the actual install process (making them run fast).  To avoid adding too much time to VS setup, though, we don’t NGEN all assemblies immediately – and instead will NGEN the rest in the background when your machine is idle.  Until it finishes NGENing the assemblies they will be JIT'd to native code the first time they are used in a process – which for large assemblies can sometimes cause a slight performance hit.

If you run into this you can manually force all assemblies to be NGEN’d to native code immediately (and not just wait till the machine is idle) by launching the Visual Studio command line prompt from the Windows Start Menu (Microsoft Visual Studio 2010->Visual Studio Tools->Visual Studio Command Prompt).  Within the command prompt type “Ngen executequeueditems” – this will cause everything to be NGEN’d immediately.