Is it normal for Autovacuum running continuously?

I notice in my postgres logfile that autovacuum is running every minute.
The server had no overnight activity, yet this has been going on for days.
Overall performance of the database seems to be slow, and I assume that it
is related.

I'm running 8.1.4 on Redhat EL4.3.

Is this normal for autovacuum?

Here is the tail of the log (even though it doesn't say much):

[4836---2007-02-13 08:28:28.194 EST]LOG: autovacuum: processing database
"xx"
[4859---2007-02-13 08:29:28.200 EST]LOG: autovacuum: processing database
"xx"
[4863---2007-02-13 08:30:28.196 EST]LOG: autovacuum: processing database
"xx"
[4865---2007-02-13 08:31:28.191 EST]LOG: autovacuum: processing database
"xx"
[4870---2007-02-13 08:32:28.201 EST]LOG: autovacuum: processing database
"xx"
[4916---2007-02-13 08:34:23.555 EST]LOG: autovacuum: processing database
"xx"
[4921---2007-02-13 08:35:23.550 EST]LOG: autovacuum: processing database
"xx"
[4923---2007-02-13 08:36:23.555 EST]LOG: autovacuum: processing database
"xx"
[4959---2007-02-13 08:37:27.169 EST]LOG: autovacuum: processing database
"xx"
[4965---2007-02-13 08:38:27.165 EST]LOG: autovacuum: processing database
"xx"
[5057---2007-02-13 08:39:27.178 EST]LOG: autovacuum: processing database
"xx"

Thanks,

Mike