Postgres slower than MS ACCESS

I am running some simple queries to benchmark Postgres 8.1 against MS Access
and Postgres is 2 to 3 times slower that Access.

Hardware:

Dell Optiplex GX280

P4 3.20 GHz

3GB RAM

Windows XP SP1

Database has one table with 1.2 million rows

Query:

UPDATE ntdn SET gha=area/10000

I could post the EXPLAIN ANALYZE results but its 4,000+ lines long

I've run various tests on a number of Postgres parameters; none of which
have come close to Access' time of 5.00 min. Postgres times range between
24 min and 121 min.

Some of the Postgres variables and ranges I've tested.

work_mem: 1,000 to 2,000,000

temp_buffers: 1,000 to 10,000

shared_buffers: 1,000 to 64,000

sort_mem: 1,024,000

fsync on / off

Why does Access run so much faster? How can I get Postgres to run as fast
as Access?

Thanks,

Jay