Here are all of the original opening theme songs (that appear on the DVDs)

The unaired pilot with John Posey
http://www.youtube.com/watch?v=Rs61cnX7pvg

Season 1
http://www.youtube.com/watch?v=Q4G-i1WImoU&mode=related&search=

Season 2 (Used in the episode "Cutting it Close" only.)
http://www.youtube.com/watch?v=hJB2Bw2PEYU

Season 2 (The other 21 episodes)
http://www.youtube.com/watch?v=JuKoRPogxl4

Season 3 (Used in 6 select episodes)
http://www.youtube.com/watch?v=9v4apMYyQ5w

Season 3 (The other 18 episodes w/ Lori Loughlin)
http://www.youtube.com/watch?v=QYjRzZIjj8w

Season 3 (The other 18 episodes w/o Lori Loughlin)
http://www.youtube.com/watch?v=fY9F35yRjxY&mode=related&search=

Season 4 (Used in the first four episodes of season 5)
http://www.youtube.com/watch?v=XkVEDP3zpK4

Season 5 (Last 22 episodes)
http://www.youtube.com/watch?v=wiba6TU-nRY

Season 6
http://www.youtube.com/watch?v=diHDR3hZb2Y

Season 7
http://www.youtube.com/watch?v=ia353fC2DDE&mode=related&search=

Season 8
http://www.youtube.com/watch?v=zjehlbxlG3M&mode=related&search=