how to silently install dotnetfx35.exe when it 's not found in target machine.

I am using VS 2008 setup project to create my setup, I included dotnetfx35.exe into my setup, how to silently install dotnetfx35.exe when it 's not found in target machine.

please help

thanks & regards

Wouldn't that be unethical? I mean I wouldn't want something to be installed on my laptop if I am not aware of it?

One more point. AFAIK and I could be wrong but installing dotnetfx35.exe will not work till you uninstall earlier pre-release versions of .NET Framework 3.5