Understanding 'could not read block'

Hi,

I have a table of about 693 million rows (80gb) of position data (standard
object,timestamp,position,etc).

Every time I try to build some statistics by creating a table, such as:
> create table pos_stats1 as
> select id,year,month,count(1) from positions group by id,year,month;

I get an error:
> ERROR: could not read block 8519713 of temporary file: Permission denied

I get a similar error sometimes on indexes. What problem am I hitting here?
The drive and permissions all seem fine & the query runs for a long time
before hitting this error. I've turned off virus scan & it seems to have no
effect.

Thanks.

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Windows Server 2003
PostgreSQL Version 8.3.0
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