I was trying to use Dotfuscator Community Edition, and I got a build error saying “Couldn’t load external type because its assembly can’t be found: System.Windows.Forms.Form,System.Windows.Forms, Version=1.0.5000.0”. I think this may have been because it was the Dotfuscator from Visual Studio 2005, although I’m not certain – the path is C:\Program Files (x86)\Microsoft Visual Studio 8\Application\PreEmptive Solutions\Dotfuscator Community Edition. So I switched over to the one in C:\Program Files (x86)\Microsoft Visual Studio 10.0\PreEmptive Solutions\Dotfuscator Community Edition, and the error went away.
Like I said, I think it was from VS 2005. But I use both VS 2005 and VS 2010, so maybe the VS 2010 install, or some update, wiped the .NET 1.0 assemblies. I only have .NET 220.127.116.11 and 18.104.22.168 versions of that assembly.