Speed issues with update

I'm using 'PostgreSQL 7.0.2 on i386-unknown-freebsdelf4.0, compiled by
cc'.

The problem is that the following statement takes approx 3hrs on a table
with approx 136,000 records.

update table_name set state = 10 where pid is null;

pid is indexed. The result from an explain on the statement is as
follows...

Seq Scan on table_name (cost=0.00..8668.56 rows=45348 width=310)

When the query is running, disk io is minimal, but cpu is maxed.

Can anyone suggest a solution or a way of finding where pg is spending all
the cpu time?

Thanks

Loz