Looking for the Office install folder.

// Taken from https://support.microsoft.com/en-us/kb/240794

RegistryKey[] keys;
if (IntPtr.Size == 8) {
    keys = new[] {
        RegistryKey.OpenBaseKey(Microsoft.Win32.RegistryHive.LocalMachine, RegistryView.Registry32),
        RegistryKey.OpenBaseKey(Microsoft.Win32.RegistryHive.LocalMachine, RegistryView.Registry64),
    };
}
else {
    keys = new[] {
        RegistryKey.OpenBaseKey(Microsoft.Win32.RegistryHive.LocalMachine, RegistryView.Default)
    };
}

const string excelProgId = "Excel.Application";
foreach(var key in keys) {
    var excelClassIdPath = string.Format(@"Software\Classes\{0}\CLSID", excelProgId);
    var excelClassIdKey = key.OpenSubKey(excelClassIdPath);
    if (excelClassIdKey == null) continue;
    var excelClassId = excelClassIdKey.GetValue(null);
    var excelLocationPath = string.Format(@"HKEY_LOCAL_MACHINE\Software\Classes\CLSID\{0}\LocalServer32", excelClassId);
    var excelLocation = key.OpenSubKey(excelLocationPath).GetValue(null);

    new {
        Name = "Excel",
        ProgId = excelProgId,
        ClassId = excelClassId,
        Path = excelLocation
    }.Dump();
    break;
}