Well let me know how things go once you move your movies onto the ext Hard Drive, that should free up a lot of space. Also if your photos that you said you'd moved off onto the ext HD in your first post, are all accounted for then you shouldn't have any photos OR movies taking up space on your MB Air.
You should have about 40GB free space then ...
Run Omnidisk Sweeper & let me know the results when this has been done.