backup and wal files

Hello to all ...
I'm trying to install a backup system for my Postgresql 8.1.1 (on windows xp) using 3 wal files.

So i change these parameters in postgresql.conf
checkpoint_segments = 3
archive_command = 'copy "%p" "D:/PostgreSQL/backup/%f"'

I stop the database service and then restart it ...

Before restart i have 1 wal now i have 1 wal file ...

I start to generate data to populate wal files and postgres start to generate new wal files and copy them to the destination folder.

The problem is that files number don't stop to increase, now i have 10 wal files in pg_xlog and 9 files in backup folder ...
why the backup files don't be deleted ?

thanks a lot in advance ...

Fabio