PG 8 Out of memory issuing queries

Hi!

Hope anybody can help me with this issue. I have a fairly large table
(~75 million rows) that where imported without pk. Tried to create a pk
on a supposed unique field, but failed with duplicates. So i issued:
select oldid from table group by oldid having count(*) > 1
and it fails with Out of memory. Tried to create a new pk using: alter
table <table> add newid serial primary key; and it also fails with out
of memory error.

Are there any settings on postgresql.conf that I may change to prevent
this error?

This is on Windows XP profesional with Postgresql 8.0


TIA,

--
Werner Bohl <WernerBohl@infutor.com>
Infutor de Costa Rica S.A.