You can get the current identity of the user under which the current thread is running (not necessarily the logged in user) using WindowsIdentity.GetCurrent(). Alternatively you can get the logged in user name via the Environment.UserName property. It is not guaranteed to be the user running the current process however.
There is no Windows API to get a user's password as passwords aren't stored in Windows.
Have a look on
Reset Windows Administrator Account Password in C#[
^]