How to get nstalled application Path in AfterInstall Event of installer class ?

Hi Experts,
   I have to register some assemblies in GAC utility after installing my application.I wrote it in afterInstall event of Installer Class as follows :
    p.GacInstall(@"C:\Documents and Settings\mathiks.ISS140\Desktop\MyApp\.IEPlugin.dll");
It works wells.Here I hardcoded path; but, it should be from installation patht. how to get Installation path in afterinstall event ?
Cheers,
Mathi.
P.S.:While I am getting Application.StartupPath in installer class , It gives C:\Windows(MSI path)Start Free Trial