How to purge old files from pg_xlogs directory
Firstly PG 8.4 is no longer supported (since July 2014), you should move your databases to a supported version as soon as possible.

The WAL configuration settings for the 8.4 version are documented here:

https://www.postgresql.org/docs/8.4/static/runtime-config-wal.html

In particular checkpoint_segments control the number of WAL files.

You must not delete WAL files, either manually or with a cron job or with archive_command. This would definitely corrupt your database.

WAL files may grow in number if archive_command is not working or lagging behind.