Primary Key

I have a table (5 columns) with approximately 670 million rows. It has had
an index (unique) on an inet column from the beginning. Today I added a
primary key constraint based on the same column thinking that since it
already had an index, this would be a relatively quick operation. That does
not appear to be case. It has gone into a "not responding" status for an
hour or so now. As a point of reference, I'm using 9.3 on a 64 bit Windows
Server 2008 (32 GB ram) and inserts so far have taken 6 1/2 - 7 minutes for
each batch of 16.7 million rows.

Other than not creating the primary key at the beginning, did I do anything
wrong? and can I reasonably expect the current operation to finish?

Joey