This will give you the current executable directory in .Net:

   System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetExecutingAssembly().Location);

While working on some installer code I ran into a problem where my C# code was called by msiexec.exe on an x64 operating system.  Msiexec.exe runs as a 32 bit process on 64 bit systems so using (IntPtr.Size == 8) or IsWow64Process won't work, the solution is to use GetSystemWow64Directory, if it returns 0 it fails and you are running on a 32 bit system, if it's non zero you are running on a 64 bit system.

public static bool Is64BitOs()
{
     StringBuilder path = new StringBuilder(256);

     int result = GetSystemWow64Directory(path, (uint)path.Length);