Error when inserting millions of records

Me and a friend are testing some operations for a search application in
PostgreSQL. In version 8.1 my friend tried to copy the result of a join
into another table, about 45.000.000 (small) records. This is the
result:

testsearch=> insert into t_documentword2 (SELECT document_id,
t_word2.id,
frequency from t_documentword, t_word2 where t_documentword.word =
t_word2.word);
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

He got the same result in version 7.5.15.

Anyone any ideas what this could be?