To get arc running on Windows I downloaded DrScheme (http://download.plt-scheme.org/drscheme/), which includes MzScheme. After installing DrScheme launch the application. When first using DrScheme the IDE will ask you to select a language in the lower left hand corner. I chose PLT -> Textual (MzScheme, includes R5RS). Then go to File -> Open and navigate to as.scm in your arc directory (it doesn't matter where the arc files are located on your system). Click Run at the top of the IDE and the Arc prompt should start up.
