Hi,
On my web site I have a NSIS_HDR add-on for my preprocessor (all free) which simplifies creation of consistant NSI files and takes care of all uninstall issues automatically. This is for people who do NOT want a GUI but want more power.
You might not want to use the add-on but use the preprocessor itself as PPWIZARD it is the most powerful preprocessor available and so can do heaps more than the NSIS preprocessor.
If interested have a look at:
http://www.labyrinth.net.au/~dbareis/ppwizard.htm
Bye
Dennis