strange autovacuum behaviour

Hi All,
I have a couple of questions about autovacuum/vacuum behavior.

On my production system, I set autovacuum ON since it's pretty hard to
me find a good timeframe for vacuum (the system is pretty busy over the day)
Last week the system was really slow and running vacuum manually the
performance was really improved.
So now I would like to understand why autovacuum did not work as
expected (at least for me). Any ideas ?

Other question.
Yesterday I noticed another strange thing. autovacuum was off since the
aforementioned issue and a vacuum scheduled at 06:00 UTC.
Checking pg_stat_all_tables (last_autovacuum field) I saw that at 05:30
UTC an autovacuum was performed for almost all the tables. After 30 min
(at 06:00) vacuum started and for about 2 hours, vacuum and autovacuum
were running simultaneously.

Why autovacuum (that was OFF) started ? For preventing transaction ID
wraparound ? Is it right that in such case all the tables are
(auto)vacuumed ?

My database is about 35 GB and the most updated table (in terms of
insert/delete/update) is indeed the biggest one (3.6 GB of data, 3.2 GB
primary key index, 1.5 GB another index) and I'm using postgres 8.2.9.

Thanks in advance for any suggestion.

Cheers,
ste