The only other reasons I can think of is, the operating system somehow has a lock on it, so iTunes can't add it. Vista is notorious for weirding out file permissions.
How To Quickie: Vista Permissions
http://paulstamatiou.com/2007/02/09/how-to-quickie-vista-permissions
How to Take Ownership and Grant Permissions in Windows Vista
http://www.mydigitallife.info/2007/05/25/how-to-take-ownership-and-grant-permiss ions-in-windows-vista/
How to Disable UAC
Click Start/ type in the Search bar msconfig without quotes and hit ENTER... In the System Configuration window that will pop-up, select the TOOLS Tab. Scroll down the list and look for Disable UAC. Select it and click launch. Close all open windows and reboot...
Also, are the movies on a SATA drive, or is it IDE?