Wrong column default values

Hi,

I have installed Postgres 8.3.8 on Fedora Linux. If I create a table,
the default values will be set by the database to

NULL::timestamp without time zone

for a timezone column and to

NULL::character varying

for a varchar column. How I can prevent such a behaviour? Both column
defaults should be NULL. I have read some posts about this point, it
seems to be a problem of a so called "acts_as_tsearch plugin", but I'm
not familiar enough with Postgres to know, what it means. Any other
detailled solutions?

Thanks
Andre