Simple UPDATE runs forever

The command

UPDATE dok SET krdokumnr=NULL WHERE krdokumnr NOT in (select dokumnr from
dok);

runs forever. Postgres.exe process takes 90% of CPU time, disk LED is
flashing.
Platform: Win XP Prof SP2, Postgres 8
dok table has only 7651 rows
Killing client application does not help. Only killing postgres.exe process
stops computer activity.

CREATE TABLE ou1.dok
(
doktyyp char(1),
dokumnr numeric(12) NOT NULL DEFAULT
nextval('"ou1".dok_dokumnr_seq'::text),
krdokumnr numeric(12),
... a lot of other fields
CONSTRAINT dok_pkey PRIMARY KEY (dokumnr)
)
WITHOUT OIDS;

any idea ?

Andrus.