From NSIS Wiki
Nsislog.zip (23 KB)
With this plugin you can write some text to a textfile at install time.
Section "Log test" ... do something ... nsislog::log "$INSTDIR\install.log" "before registering my dll" RegDll "$INSTDIR\mydll.dll" nsislog::log "$INSTDIR\install.log" "after registering my dll" ... do something more ... SectionEnd
At any time you can use different files for your logs. After the call, your text is written and the file is closed.
Note: Very useful to debug a installation without a compilation of the NSIS compiler.
Hope its useful.