Very slow query, Help please!

Hi !

Help me please to resolv my problem.

I have two tables . One of them is large (say 100000 records)
with unique index on "ID"
and the second table (5000 records ) which i have to insert
into the first table , but the second table have much records,
which have many duplicate values in "ID" . and this is
the query which i used for insertion:

insert into LTable select * from STable
where ID not in (select ID from LTable )

this query takes much time. and moreover - for big tables
i couldn't got result of query for about an hour, it looks
like so that it is die...

May be there is anoter way for insertion ?

Thanks for any suggestion!

Igor