ERROR: could not open file "pg_subtrans...

I have Postgres 8.1.3 running on Windows XP. I keep seeing this error in the log files:

2007-09-29 10:11:29 ERROR: could not access status of transaction 5745045
2007-09-29 10:11:29 DETAIL: could not open file "pg_subtrans/0057": Invalid argument

I am also getting the following in the Windows Applcation event log:

FATAL: could not reattach to shared memory (key=5432001, addr=01C80000): Invalid argument

It is causing quite a few problems. I have searched around and have not found to much on it. I have
the same database/hardware set-up on two other machines and they work fine. I have not tried to
reinstall postgres yet. I know it only happens when three particular tables are being used. I have
droped these tables and created new ones and I am still geting the error. These tables get a lot of
insert, updates and deletes. The three table schemas are:

CREATE TABLE scanorder
(
scn_pk int4 NOT NULL DEFAULT nextval(('"scanorder_scn_pk_seq"'::text)::regclass),
scn_usr_fk int4,
scn_str_fk int4,
scn_ogp_fk int4,
scn_date timestamp,
scn_processed_date timestamp,
scn_report bool NOT NULL DEFAULT false,
scn_processed bool NOT NULL DEFAULT false,
scn_sent bool NOT NULL DEFAULT false,
scn_sending bool NOT NULL DEFAULT false,
scn_total float4,
scn_time_changed timestamp,
CONSTRAINT scanorder_pkey PRIMARY KEY (scn_pk)
)

CREATE TABLE order_
(
ord_pk int4 NOT NULL DEFAULT nextval(('"order_ord_pk_seq"'::text)::regclass),
ord_usr_fk int4,
ord_str_fk int4,
ord_scn_fk int4,
ord_ven_fk int4,
ord_ogp_fk int4,
ord_date timestamp,
ord_processed_date timestamp,
ord_delivered_date timestamp,
ord_received_date timestamp,
ord_po_number char(25),
ord_invoice_number char(25),
ord_account_number char(30),
ord_vendor_code char(10),
ord_vendor_name char(25),
ord_report bool NOT NULL DEFAULT false,
ord_processed bool NOT NULL DEFAULT false,
ord_delivered bool NOT NULL DEFAULT false,
ord_received bool NOT NULL DEFAULT false,
ord_time_changed timestamp,
ord_memo char(30),
ord_sent_to char(25),
CONSTRAINT order_pkey PRIMARY KEY (ord_pk),
CONSTRAINT order_scanorder_ord_scn_fk FOREIGN KEY (ord_scn_fk)
REFERENCES scanorder (scn_pk) MATCH SIMPLE
ON UPDATE RESTRICT ON DELETE CASCADE
)

CREATE TABLE order_
(
ord_pk int4 NOT NULL DEFAULT nextval(('"order_ord_pk_seq"'::text)::regclass),
ord_usr_fk int4,
ord_str_fk int4,
ord_scn_fk int4,
ord_ven_fk int4,
ord_ogp_fk int4,
ord_date timestamp,
ord_processed_date timestamp,
ord_delivered_date timestamp,
ord_received_date timestamp,
ord_po_number char(25),
ord_invoice_number char(25),
ord_account_number char(30),
ord_vendor_code char(10),
ord_vendor_name char(25),
ord_report bool NOT NULL DEFAULT false,
ord_processed bool NOT NULL DEFAULT false,
ord_delivered bool NOT NULL DEFAULT false,
ord_received bool NOT NULL DEFAULT false,
ord_time_changed timestamp,
ord_memo char(30),
ord_sent_to char(25),
CONSTRAINT order_pkey PRIMARY KEY (ord_pk),
CONSTRAINT order_scanorder_ord_scn_fk FOREIGN KEY (ord_scn_fk)
REFERENCES scanorder (scn_pk) MATCH SIMPLE
ON UPDATE RESTRICT ON DELETE CASCADE
)

I am a complete novice with postgres so bare with me.

--
Thanks,

Warren Bell