Seq scan

Hi everybody.
Im trying to execute a simple UPDATE query on a table with 450000 rows
using an index in the where clause.

Example:
UPDATE table SET field = null WHERE field = 12345
( NOTICE: Field has an index )

But when I use the EXPLAIN command it always tells me that Postgres is
using Seq Scan, even if I disable Seq Scan in the postgresql.conf

The PostgreSQL is running on a Pentium III 900 Mhz, 512 MB RAM, HD SCSI
18 Gb with a RedHat Linux 7.3 installed.
Anyone can help me?
Thanks in advance,

Diogo Biazus
diogo@ikono.com.br
Ikono Sistemas & Automação
www.ikono.com.br