Well, it's not going to matter anyway because I can't see any way to add a column to a system table. I figured you would be able to do this but adding a column is not an option for system tables and I tried to do it via a SQL statement and it doesn't like that either. I don't know what that column does or why it's missing. Can you check on the other computer that you did the backup from and see if that field is in that table?
Another option is to try to do the backup directly from pgAdminIII and see if that gives any different results. Failing that, backing up the entire c:\program files\postgresql\8.3\data folder would also provide a backup except it wouldn't be just a single compressed file containing the backup but rather that entire folder...and sub-folders.