Comment 1 for bug 582648

Well, thats quite normal.
If you absolutely want your build output in Program Files, you should run msbuild/hsbuild with admin privileges.

I would recommend you to place your .modules file and/or source code somewhere else.
Also, as a side note, use HSBUILD_PREFIX_PATH environment variable to set output root path. The HSBuild.exe tool does this btw.