With Windows 8 released to MSDN and TechNet subscribers today, I thought it'd be a good idea to give a short guide for installing SharpDevelop on a blank Windows 8 installation - because it will fail with an error message (although the screenshot is for SharpDevelop 4.3, the same applies for 4.2):
The easiest way to fix this is to install .NET 3.5 (used by lots of other applications / drivers, thus if you don't install SharpDevelop first thing on Windows 8, it might just install right away because somebody else auto-installed this dependency).
You are looking for "Turn Windows features on or off" - this will open the "Windows Features" dialog on the desktop:
Make sure to check ".NET Framework 3.5", click OK, let it connect to Windows Update and finish the installation. Once done, you can go back to installing SharpDevelop, as it now has all required dependencies!
Note You can alternatively install just the VC++ 2008 SP1 runtime.