SELECT statement takes 10 minutes to answer

Hi,

Iam using PostgreSQL 7.4.5 on a linux machine, and have the problem
that an easy "SELECT field1, field2, field3 FROM a_table"-statement
takes 10 minutes to give an answer. The table has 750.000 datarows. I
also made an VACUUM a_table before the statement, but it doesnt help at
all. The table has an index to a timestamp field..
What could be the problem, or is it normal that postgresql cant handle
with so many data in an acceptable response-time?

Regards,
M.Korkmaz