C# searching and getting Absolute Path of Software

I am new to C# and I want to create a little Autorun Manager in a Console Application. To do that I want to read from the console a keyword like "Steam" and my programm should search on "C:\" and in all subdirectories for a folder called like my keyword but as I say I have no Idea how I can search in all subdirectories.

Thats a little code sample with steam how I would write in registry

//filePath would be the Path of the .EXE file that was found    
string filePath = "\"C:\\Programme\\Steam\\Steam.exe\""; 
string autostartPath = @"Software\Microsoft\Windows\CurrentVersion\Run\";
RegistryKey autostart = Registry.CurrentUser.OpenSubKey(autostartPath, true);
autostart.SetValue("Steam", filePath);

If someone know how to search in all subdirectories and find the correct .exe file.

I would be really grateful for a code sample.

Thanks in advance