I want to copy some tables!

hello,
I've been running my site for about a year and the database is getting larger and larger .. there's a lot of table that I don't really know what they are for ..it's a mess!!

I came up with the idea of making a new fresh copy of drupal and installing new fresh modules ... I copied `node` and `user` tables but this didn't give me a real clean result

does anyone out there knows how to copy the users, nodes and comments to a fresh copy of drupal having least problems

plz help ..
Tammamtu