You can use an external command line tool for unzipping your archive, see here(http://stahlworks.com/dev/index.php?tool=zipunzip) for example. Put it in your [Files] section:

[Files]
Source: "UNZIP.EXE"; DestDir: "{tmp}"; Flags: deleteafterinstall
Then call it in your [Run] section, like this:

[Run]
Filename: "{tmp}\UNZIP.EXE"; Parameters: "{tmp}\ZipFile.ZIP -d C:\TargetDir"