using System;
using System.Windows.Forms;

namespace UserLogonInfo
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
      DateTime? lastLogon; bool ok;

      ok = UserInfos.GetLastLogon_withDirectoryEntry(Environment.MachineName,
         Environment.UserName, out lastLogon); // langsam

      //ok = UserInfos.GetLastLogonTime_withAPI(Environment.MachineName,
      // Environment.UserName, out lastLogon); // schnell

      if (lastLogon.HasValue)
        MessageBox.Show("Letzte Logon-Zeit: " + lastLogon.ToString());
      else
        MessageBox.Show("Logon-Info nicht vorhanden");
    }
  }
}

UserInfos.cs:

using System;
using System.Runtime.InteropServices;
using LPWSTR = System.String;
using DWORD = System.Int32;
using System.DirectoryServices;

class UserInfos
{
  #region PInvoke Deklarationen

  /// <summary>
  /// [NetUserGetInfo Function (Windows)]
  /// http://msdn.microsoft.com/en-us/library/aa370654(VS.85).aspx
  /// </summary>
  [DllImport("Netapi32.dll", CharSet = CharSet.Unicode, ExactSpelling = true)]
  private extern static int NetUserGetInfo(
      [MarshalAs(UnmanagedType.LPWStr)] string serverName,
      [MarshalAs(UnmanagedType.LPWStr)] string userName,
      UserInfoNr level, out IntPtr bufPtr);

  [DllImport("Netapi32.dll", SetLastError = true)]
  private extern static int NetApiBufferFree(IntPtr buffer);

  /// <summary>
  /// [USER_INFO_4 Structure (Windows)]
  /// http://msdn.microsoft.com/en-us/library/aa371339(VS.85).aspx
  /// </summary>
  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
  public struct USER_INFO_4
  {
    public LPWSTR name; public LPWSTR password;
    public DWORD password_age; public DWORD priv;
    public LPWSTR home_dir; public LPWSTR comment;
    public DWORD flags; public LPWSTR script_path;
    public DWORD auth_flags; public LPWSTR full_name;
    public LPWSTR usr_comment; public LPWSTR parms;
    public LPWSTR workstations; public DWORD last_logon;
    public DWORD last_logoff; public DWORD acct_expires;
    public DWORD max_storage; public DWORD units_per_week;
    public IntPtr logon_hours;      //PBYTE
    
    public DWORD bad_pw_count; public DWORD num_logons;
    public LPWSTR logon_server; public DWORD country_code;
    public DWORD code_page; public IntPtr user_sid;   //PSID
    public DWORD primary_group_id; public LPWSTR profile;
    public LPWSTR home_dir_drive; public DWORD password_expired;
  }

  enum UserInfoNr : int
  {
    /// <summary>[USER_INFO_4 Structure (Windows)]</summary>
    Nr_4 = 4
  }

  #endregion

  /// <summary>Wird zurückgegeben, wenn NetUserGetInfo erfolgreich ist.</summary>
  const int NetReturnSuccess = 0;

  /// <summary>Letzte Logon-Zeit des Users.
  /// (recht schnell im Gegensatz zu der Methode mit DirectoryEntry)</summary>
  static public bool GetLastLogonTime_withAPI(string machineName, string accountName, out DateTime? lastLogon)
  {
    USER_INFO_4 userInfo4 = new USER_INFO_4();
    IntPtr uinfo4; lastLogon = null;
    int ret = NetUserGetInfo(machineName, accountName, UserInfoNr.Nr_4, out uinfo4);
    if (ret == NetReturnSuccess)
    {
      userInfo4 = (USER_INFO_4)Marshal.PtrToStructure(uinfo4, typeof(USER_INFO_4));
      lastLogon = new DateTime(1970, 1, 1).AddSeconds(userInfo4.last_logon).ToLocalTime();
    }
    NetApiBufferFree(uinfo4);
    uinfo4 = IntPtr.Zero;
    return (ret == NetReturnSuccess);
  }

  /// <summary>LastLogon über DirectoryEntry (ziemlich langsam)
  /// Geht aber auch für lokale Maschinen</summary>
  static public bool GetLastLogon_withDirectoryEntry(string machineName,
    string userName, out DateTime? lastLogon)
  {
    lastLogon = null;
    try
    {
      string fullName = machineName + "/" + userName;
      DirectoryEntry user = new DirectoryEntry(@"WinNT://" + fullName);
      lastLogon = (DateTime)user.Properties["LastLogin"].Value;
    }
    catch { }
    return false;
  }
}