Why autovacuum in not running

I installed postgres using windows installer and added the following lines
to end of postgresql.conf :

listen_addresses = '*'
log_destination = 'stderr'
redirect_stderr = on
stats_start_collector = on
stats_row_level = on
autovacuum = on
shared_buffers= 15000 #
log_line_prefix='\n%t %u %d %h %p %i %l %x '
log_min_duration_statement = 20000
log_min_messages = warning

I re-loaded configuration. Log file complains that share_buffers setting is
ignored.

So i assume that autovacuum is running.
However log files do not contain any lines about vacuuming.
After running analyze manually query runs a lot faster.
So I think that autovacuum is not runnning in server.
Why ?
How to make it to run ?
How to check is autovacuum running or not ?

Andrus.

"PostgreSQL 8.2.4 on i686-pc-mingw32, compiled by GCC gcc.exe (GCC) 3.4.2
(mingw-special)"