vacuum and serial primary keys

Hello -

I am using Postgresql 7.1.3 and I have deleted 3000 rows from a table
(with DELETE).
Than I used vacuum <mytable> to actually delete the rows markes as deleted
by the DELETE command..
The trouble is that the "counter" for the serial primary key (ID field)
wasn't reset. So now althought I have only 2 rows in my table they have
the ID 3001 and 3002.

Is this normal ? If not could someone please explain me how could  I reset
the "counter" to ignore the deleted rows ?

Thank you very much
Carmen