Running Vista Home Basic SP2, I was getting the "System.IO.FileNotFoundException: Could not load file or assembly 'WindowsBase, Version=3.0.0.0..." error message when trying to install Paint.NET 3.5.5. I tried performing a Repair on .NET Framework 3.5 SP1, but that didn't help. A quick Google search found this thread, I turned on the XPS Viewer feature, and the Paint.NET 3.5.5 install went smoothly. Thanks to alesx for that solution!