NOT IN query issues

I'm trying to run a DELETE query using NOT IN and it's causing me some
problems.

Here is the query that I want to run:

    delete from sharinfo where file__no not in (select file__no from
allforms);

When I explain this I get:

    Seq Scan on sharinfo  (cost=0.00..2225204.88 rows=5239 width=6)
      SubPlan
        ->  Seq Scan on allforms  (cost=0.00..424.69 rows=8669 width=12)

Which while a very large number does seem to work.  But when I run the query
nothing appears to happen, I've left this query running for hours with no
response back.  Is there a better way to run a query like this (deleting all
rows that don't appear in a second table)?

Thanks,
Rob