Expensive query

I have two tables, structured as:

users:

id varchar(70) not null unique

events:

userid varchar(70) not null,
code char(10) not null,
when datetime not null

The query I need to perform answers the question, "Which users do NOT
have a particular event (selected by code), and which do not have ANY
event for the last day?" The query I use is:

select id from users
where
id not in (
select unique id from events
where code = 'some code'
)
and
id not in (
select unique id from events
where age('now',when) <= '1 day'
);

This query is *very* expensive. With 10,000 users and 40,000 events,
it can take up to 20 minutes (!) to execute, and the postgres process
grows to 40 megabytes (!!) of memory (on a Pentium II system running
BSDI). This seems surprising, since each of the subqueries only needs
to be evaluated once, rather than once per row of users. Is there a
way to reformulate this query to make it less expensive?