You can't download movies a second time, unlike some other media. You could have transferred them from the old computer along with other files (did you do that?).
Downloading past purchases from the App Store, iBookstore, and iTunes Store - http://support.apple.com/kb/ht2519 - enabled with iTunes 10.3 and newer; not available in all countries; only apps, books (not audiobooks), music, and t.v. shows (not movies). Discontinued items not available. For items not included in the iCloud list, or locations or computer systems where iCloud is not (yet?) available, you only get one download per fee paid. Apple notes it is your responsibility to back up your purchases.