From C# code:

I have com add-in to outlook 2003, code in c#.
I want to check if this add-in already  installed in this computer.

private void isOutlook()
{
Microsoft.Win32.RegistryKey key =
Microsoft.Win32.Registry.LocalMachine.OpenSubKey("Software\\microsoft\\windows\\currentversion\\app paths\\OUTLOOK.EXE");
string path = (string)key.GetValue("Path"); if( path != null)// if have
outlook do the installing com add-in    
//if this com add-in not yet installed      

System.Diagnostics.Process.Start("C:\\MatarotToolbar\\MatarotToolbarSetup\\Debug\\Setup.Exe");
}