We have recently seen some Win32:Dropper-gen [Drp] false positives appear for compressed packer files, here INNO.
Gen meaning it is a generic find, and therefore also meaning more false positive prone.
This apparently is caused by the «runtime packed» a.k.a. «execution compression» feature of the compiler (or linker?)
A kind of similar INNO_set.up false positive issue was also flagged in June 2007.
If it is solved with a coming update, you will be certain it has been a false positive, which is rather likely i.m.o.

polonus