How to sort complete sets from box of unsorted LEGO
First thing to do would be to get together any building instructions you have and attempt to build those sets one at a time noting any missing elements as you go.

Once this is complete and if you still have unsorted pieces, you could use a website such as Bricklink.com to try to identify the sets and then use Peeron.com to source the building instructions for that set.

To begin this process, try to select an odd looking piece that is uncommon in your collection. Use Bricklink to try to identify the piece and then click through to the catalog entry for that piece to see the list of sets that piece is included in. You can follow the links to get an inventory of the selected set and then see how many of the required pieces you have in the unsorted box.

This is not an exact science and will require a fair amount of trial-and-error, not to mention time, but these two websites are two of the best LEGO resources available for such a task.