Windows API function:
BOOL GetUserName(
LPTSTR lpBuffer, // address of name buffer
LPDWORD nSize // address of size of name buffer
);
Delphi example:
procedure X;
var
USize : DWORD;
pUName : pchar;
sUName:string;
begin
USize := 30;
getmem(pUName, USize);
if GetUserName(pUName, USize) then
sUserName := StrPas(pUName)
else
sUserName := 'Unknown';
freemem( pUName, USize );
end;