Avast is detecting NSIS 3.0b0 as a false positive. However, NSIS 2.46 downloads fine, I have submited the file so it can be fixed, please fix this asap, as a developer it is highly inconvenient to have Avast detect an install maker tool as a false positive. The source code for NSIS 3.0b0 is also conveniently made available. Please fix this issue asap.
Download Url: http://nsis.sourceforge.net/Download