I was a bit hasty sending my previous posting. Here is the correct SQL-statement:
SELECT t1.id, t1.date, t1.ip, t1.user, IIf(t1.user<>'','',(SELECT top 1 t2.user FROM counter t2 WHERE t2.ip = t1.ip AND t2.user<>'' ORDER BY id DESC)) AS lookup FROM counter t1 ORDER BY id DESC"
I forgot to...