If you are already have Visual Studio (rather than, say, MinGW) you can simply download the source for LLVM and follow the instructions at http://clang.llvm.org/get_started.html to compile it. Clang 3.4.1 also has a pre-built release for Windows, get it from http://llvm.org/releases/download.html - note that there doesn't seem to be one for 3.4.2 yet, though.