How to create "conditional section selection" in the Launcher

hi all.
I'm somehow new to this PA and NSIS world and sadly my English isn't so good to searching and questioning well . hope you understand my question correctly

----->>question :
I want to have a "conditional section" in my PortableApp launcher .

3 sections required :
--section a (that's my question)
--section b (integrating avisynth or whatever apps to localsystem)
--section c (integrate and run main program that "needs" avisynth (or whatever apps)integrated to the local system first)

(that's my question again)
so "section a" should check availability of a file on the local system . e.g. check file "$SYSDIR\avisynth.dll"
if that file already exists , then ignore "section b" and launch "section c" .
if that file does NOT exists , then first launch "section b" and after that launch "section c"

thanks for your time and answers!