Как узнать имя пользователя?

Вариант 1:

function GetUserFromWindows: string;
var
  UserName : string;
  UserNameLen : Dword;
begin
  UserNameLen := 255;
  SetLength(userName, UserNameLen);
  if GetUserName(PChar(UserName), UserNameLen) then
    Result := Copy(UserName,1,UserNameLen - 1)
  else
    Result := '';
end;

Вариант 2:

Source: DelphiWorld 6.0 https://delphiworld.narod.ru/

function GetCurrentUserName: string;
const
  cnMaxUserNameLen = 254;
var
  sUserName: string;
  dwUserNameLen: DWORD;
begin
  dwUserNameLen := cnMaxUserNameLen - 1;
  SetLength(sUserName, cnMaxUserNameLen);
  GetUserName(PChar(sUserName), dwUserNameLen);
  SetLength(sUserName, dwUserNameLen);
  Result := sUserName;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(GetCurrentUserName);
end;

Вариант 3:

Source: https://www.swissdelphicenter.ch

function GetCurrentUserName(var CurrentUserName: string): Boolean;
var
  BufferSize: DWORD;
  pUser: PChar;
begin
  BufferSize := 0;
  GetUserName(nil, BufferSize);
  pUser := StrAlloc(BufferSize);
  try
    Result := GetUserName(pUser, BufferSize);
    CurrentUserName := StrPas(pUser);
  finally
    StrDispose(pUser);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  CurrentUserName: string;
begin
  GetCurrentUserName(CurrentUserName);
  Label1.Caption :=  CurrentUserName;
end;

{*********************************************}

{ 
  Windows NT/2000/XP: 

  The GetUserNameEx function retrieves the name of the user or other 
  security principal associated with the calling thread. 
  You can specify the format of the returned name. 
  If the thread is impersonating a client, GetUserNameEx 
  returns the name of the client. 
}

const
  NameUnknown = 0; // Unknown name type. 
  NameFullyQualifiedDN = 1;  // Fully qualified distinguished name 
  NameSamCompatible = 2; // Windows NT® 4.0 account name 
  NameDisplay = 3;  // A "friendly" display name 
  NameUniqueId = 6; // GUID string that the IIDFromString function returns 
  NameCanonical = 7;  // Complete canonical name 
  NameUserPrincipal = 8; // User principal name 
  NameCanonicalEx = 9;
  NameServicePrincipal = 10;  // Generalized service principal name 
  DNSDomainName = 11;  // DNS domain name, plus the user name 


procedure GetUserNameEx(NameFormat: DWORD;
   lpNameBuffer: LPSTR; nSize: PULONG); stdcall;
   external 'secur32.dll' Name 'GetUserNameExA';


function LoggedOnUserNameEx(fFormat: DWORD): string;
var
  UserName: array[0..250] of char;
  Size: DWORD;
begin
  Size := 250;
  GetUserNameEx(fFormat, @UserName, @Size);
  Result := UserName;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Edit1.Text := LoggedOnUserNameEx(NameSamCompatible);
end;