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.