nsis? Oh sorry, I thought I had read that you would have something to do with Windows Installer 3.1, which lead me to think it would’ve been a msi installer.
nsis is very good in creating installers with small overhead, but a pain to write proper scripts for it really.
msi isn’t much better really if you use the open source WiX stuff from Microsoft, loads of XML that – just to keep it purely a database – gets unnecessarily complicated and illogical in many places.
the best free installer I’ve seen so far is InnoSetup, which I have in use for years now. Scripts that are easy to read, useful help file, and an engine that is well thought through (e.g. allows simple registration of libraries, clear version handling, file replacement handling, good uninstaller, etc.).
the only reason I started using msi (in parallel to a innosetup installer) recently was that msi is a requirement for the Certified for Vista logo program.