Installsheild scripts can use any protection from serials to dongles but commonly it is registry/serial check. what vesion of installshiled is it anyway?

U can try installsheild script decompilers _search for one in your fav search engine. eg wisdec.exe - runs in win9x only!

open ins script look for the line which contains the nag message and invert its caller.
dont forget to fix crc - available with the app - one i quoted

enjoy.