I imported the tables from the old SQL 2000 database into the new SQL 2008 database.
That does not work as you have found it causes the loss of primary keys, forteign keys, default values and other problems. Importing objects from one db to another is the wrong way to do it.
The only correct way to do it is to make a database backup on sql 2000 and then restore it on sql 2008, or use a tool such as Red Gate Sql bundle or this free tool to script off the structure and data correctly into a script that you can run on the new db.
Hope that helps,